Invalidate best size when the bitmap or bitmap position changes. Add GetBitmapMargins

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63595 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/button.cpp

@@ -664,6 +664,7 @@ void wxButton::DoSetBitmapPosition(wxDirection dir)
         }
 
         gtk_button_set_image_position(GTK_BUTTON(m_widget), gtkpos);
+        InvalidateBestSize();
     }
 #endif // GTK+ 2.10+
 }