Fix positioning of ComboBoxEditor in wxPG (wxGTK)

Position of the combo box editor associated with properties having custom
bitmaps like e.g. ColourProperty, needs to be adjusted in order to display
edited text at the same position as the text which is displayed as
a property value prior to the editing.

src/propgrid/editors.cpp

@@ -701,6 +701,20 @@ public:
         return rect.width;
     }
 
+#if defined(__WXMSW__)
+#define wxPG_TEXTCTRLXADJUST3 0
+#elif defined(__WXGTK__)
+  #if defined(__WXGTK3__)
+  #define wxPG_TEXTCTRLXADJUST3 2
+  #else
+  #define wxPG_TEXTCTRLXADJUST3 0
+  #endif // wxGTK3/!wxGTK3
+#elif defined(__WXOSX__)
+#define wxPG_TEXTCTRLXADJUST3 0
+#else
+#define wxPG_TEXTCTRLXADJUST3 0
+#endif
+
     virtual void PositionTextCtrl( int textCtrlXAdjust,
                                    int WXUNUSED(textCtrlYAdjust) ) wxOVERRIDE
     {
@@ -709,8 +723,8 @@ public:
                           (wxPG_XBEFOREWIDGET+wxPG_CONTROL_MARGIN+1) - 1,
     #endif
         wxOwnerDrawnComboBox::PositionTextCtrl(
-            textCtrlXAdjust,
-            0 // Under MSW vertical position is already properly adjusted.
+            textCtrlXAdjust + wxPG_TEXTCTRLXADJUST3,
+            0 // Under MSW, GTK vertical position is already properly adjusted.
               // Note: This parameter is not used by other ports.
         );
     }