Allow to switch to the Simple mode, when all objects are single-parts or have support-modificators only.

+ Object's menu for Advanced and Expert mode are the same now.

+ "wxYES|wxCANCEL" style is changed to "wxOK|wxCANCEL" for MessageDialog.
The reason: wxWidgets implementation for Linux doesn't respects to "wxYES|wxCANCEL" style for MessageDialog.
In this case massage dialog has "wxYES_NO|wxCANCEL" style.

src/slic3r/GUI/GUI_App.cpp

@@ -2324,13 +2324,33 @@ ConfigOptionMode GUI_App::get_mode()
            mode == "simple" ? comSimple : comAdvanced;
 }
 
-void GUI_App::save_mode(const /*ConfigOptionMode*/int mode) 
+bool GUI_App::save_mode(const /*ConfigOptionMode*/int mode) 
 {
     const std::string mode_str = mode == comExpert ? "expert" :
                                  mode == comSimple ? "simple" : "advanced";
+
+    auto can_switch_to_simple = [](Model& model) {
+        for (const ModelObject* model_object : model.objects)
+            if (model_object->volumes.size() > 1) {
+                for (size_t i = 1; i < model_object->volumes.size(); ++i)
+                    if (!model_object->volumes[i]->is_support_modifier())
+                        return false;
+            }
+        return true;
+    };
+
+    if (mode == comSimple && !can_switch_to_simple(model())) {
+        show_info(nullptr,
+            _L("Simple mode supports manipulation with single-part object(s)\n"
+            "or object(s) with support modifiers only.") + "\n\n" +
+            _L("Please check your object list before mode changing."),
+            _L("Change application mode"));
+        return false;
+    }
     app_config->set("view_mode", mode_str);
     app_config->save(); 
     update_mode();
+    return true;
 }
 
 // Update view mode according to selected menu

src/slic3r/GUI/GUI_App.hpp

@@ -263,7 +263,7 @@ public:
 
     Tab*            get_tab(Preset::Type type);
     ConfigOptionMode get_mode();
-    void            save_mode(const /*ConfigOptionMode*/int mode) ;
+    bool            save_mode(const /*ConfigOptionMode*/int mode) ;
     void            update_mode();
 
     void            add_config_menu(wxMenuBar *menu);

src/slic3r/GUI/GUI_Factories.cpp

@@ -465,19 +465,19 @@ void MenuFactory::append_menu_item_delete(wxMenu* menu)
 
 }
 
-wxMenu* MenuFactory::append_submenu_add_generic(wxMenu* menu, ModelVolumeType type) {
+wxMenu* MenuFactory::append_submenu_add_generic(wxMenu* menu, ModelVolumeType type)
+{
     auto sub_menu = new wxMenu;
 
     const ConfigOptionMode mode = wxGetApp().get_mode();
 
-    if (mode == comExpert   && type != ModelVolumeType::INVALID ||
+    if (mode > comSimple) {
-        mode == comAdvanced && type == ModelVolumeType::MODEL_PART) {
         append_menu_item(sub_menu, wxID_ANY, _L("Load") + " " + dots, "",
             [type](wxCommandEvent&) { obj_list()->load_subobject(type); }, "", menu);
         sub_menu->AppendSeparator();
     }
 
-    if (!(type == ModelVolumeType::MODEL_PART && mode == comAdvanced))
+    //if (!(type == ModelVolumeType::MODEL_PART && mode == comAdvanced))
         for (auto& item : { L("Box"), L("Cylinder"), L("Sphere"), L("Slab") })
         {
             if (type == ModelVolumeType::INVALID && strncmp(item, "Slab", 4) == 0)
@@ -527,6 +527,7 @@ void MenuFactory::append_menu_item_add_text(wxMenu* menu, ModelVolumeType type,
         ) {
         wxString item_name = is_submenu_item ? "" : _(ADD_VOLUME_MENU_ITEMS[int(type)].first) + ": ";
         item_name += _L("Text");
+        menu->AppendSeparator();
         const std::string icon_name = is_submenu_item ? "" : ADD_VOLUME_MENU_ITEMS[int(type)].second;
         append_menu_item(menu, wxID_ANY, item_name, "", add_text, icon_name, menu);
     }

src/slic3r/GUI/Plater.cpp

@@ -2659,10 +2659,10 @@ std::vector<size_t> Plater::priv::load_files(const std::vector<fs::path>& input_
             }
             if ((wxGetApp().get_mode() == comSimple) && (type_3mf || type_any_amf) && model_has_advanced_features(model)) {
                 MessageDialog msg_dlg(q, _L("This file cannot be loaded in a simple mode. Do you want to switch to an advanced mode?")+"\n",
-                    _L("Detected advanced data"), wxICON_WARNING | wxYES | wxCANCEL);
+                    _L("Detected advanced data"), wxICON_WARNING | wxOK | wxCANCEL);
-                if (msg_dlg.ShowModal() == wxID_YES) {
+                if (msg_dlg.ShowModal() == wxID_OK) {
-                    Slic3r::GUI::wxGetApp().save_mode(comAdvanced);
+                    if (wxGetApp().save_mode(comAdvanced))
-                    view3D->set_as_dirty();
+                        view3D->set_as_dirty();
                 }
                 else
                     return obj_idxs;
@@ -5299,8 +5299,8 @@ void Plater::priv::update_after_undo_redo(const UndoRedo::Snapshot& snapshot, bo
     if (wxGetApp().get_mode() == comSimple && model_has_advanced_features(this->model)) {
         // If the user jumped to a snapshot that require user interface with advanced features, switch to the advanced mode without asking.
         // There is a little risk of surprising the user, as he already must have had the advanced or expert mode active for such a snapshot to be taken.
-        Slic3r::GUI::wxGetApp().save_mode(comAdvanced);
+        if (wxGetApp().save_mode(comAdvanced))
-        view3D->set_as_dirty();
+            view3D->set_as_dirty();
     }
 
 	// this->update() above was called with POSTPONE_VALIDATION_ERROR_MESSAGE, so that if an error message was generated when updating the back end, it would not open immediately, 

src/slic3r/GUI/wxExtensions.cpp

@@ -747,9 +747,11 @@ ModeSizer::ModeSizer(wxWindow *parent, int hgap/* = 0*/) :
 {
     SetFlexibleDirection(wxHORIZONTAL);
 
-    auto modebtnfn = [](wxCommandEvent &event, int mode_id) {
+    auto modebtnfn = [this](wxCommandEvent &event, int mode_id) {
-        Slic3r::GUI::wxGetApp().save_mode(mode_id);
+        if (Slic3r::GUI::wxGetApp().save_mode(mode_id))
-        event.Skip();
+            event.Skip();
+        else
+            SetMode(Slic3r::GUI::wxGetApp().get_mode());
     };
     
     m_mode_btns.reserve(3);