Using of wxWidgets 3.1.6 WIP: Linux/OSX specific fixes

OSX specific: Fixed get_mouse_position_in_control().
+ Use GetItemRect() to calculation of the position and size for Extruder selector

Linux specific:
* Use just 1.0 scale for wxBitmapComboboxes under GTK3 and gtk3
* GTK2 specific: use GTK2 doesn't suppost get_scale, so scale bitmap size form em_unit()

src/slic3r/GUI/OG_CustomCtrl.cpp

@@ -21,10 +21,10 @@ static bool is_point_in_rect(const wxPoint& pt, const wxRect& rect)
 
 static wxSize get_bitmap_size(const wxBitmapBundle* bmp, wxWindow* parent)
 {
-#ifdef __APPLE__
-    return bmp->GetDefaultSize();
-#else
+#ifdef __WIN32__
     return bmp->GetBitmapFor(parent).GetSize();
+#else
+    return bmp->GetDefaultSize();
 #endif
 }