Implement getters for members of wxPGWindowList

There is no reason to expose wxPGWindowList member variables. They should
be set in ctors and retrieved with getters.

src/propgrid/editors.cpp

@@ -1318,7 +1318,7 @@ wxPGWindowList wxPGChoiceAndButtonEditor::CreateControls( wxPropertyGrid* propGr
 #endif
 
     wxWindow* ch = wxPGChoiceEditor::CreateControls(propGrid,property,
-        pos,ch_sz).m_primary;
+        pos,ch_sz).GetPrimary();
 
 #ifdef __WXMSW__
     bt->Show();