Allow wxPGMultiButton to work even if primary editor control was not created in wxPGEditor::CreateControls(); Clarified related samples and documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59450 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/editors.h

@@ -534,6 +534,8 @@ public:
 
 protected:
 
+    void DoAddButton( wxWindow* button, const wxSize& sz );
+
     int GenId( int id ) const;
 
     wxArrayPtrVoid  m_buttons;