Implementation for #6216

* Implementation for #6216 - Make number keys select extruder when object treeview has focus
+ deleted unused extruder_selection()
+ Fixed notification after splitting of the solid object

* Follow up 85a10268b9 - OSX implementation
+ Added shortcuts description to the "Keyboard Shortcuts" dialog

* Workaround to use "+/-" and numbers shortcuts on Linux
+ Fixed build on Linux

* OSX specific: fixed a work of keyboard accelerators from numbers on NumPad keyboard

* KBShortcutsDialog: fixed shortcuts for "Preferences" and "Show/Hide 3Dconnexion devices settings dialog, if enabled" under osx
and "Set Printable/Unprintable" and "Set extruder" under Linux
 + OSX specific: Added minimize of the application on "Cmd+M"

* Hot-fix for 6efeb9d6b4

* Removed Linux specific workaround

src/slic3r/GUI/Search.hpp

@@ -53,7 +53,7 @@ struct Option {
     std::wstring    category;
     std::wstring    category_local;
 
-    std::string     opt_key() const { return boost::nowide::narrow(key).substr(2); }
+    std::string     opt_key() const;
 };
 
 struct FoundOption {