Implement SetShape() for wxPopupWindow in wxGTK.

Move SetShape() implementation from wxTopLevelWindow to wxNonOwnedWindow so
that wxPopupWindow, which also inherits from the latter, could use it as well.

This makes it possible to have popup windows with irregular shapes in wxGTK as
well as in wxMSW and wxOSX.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69408 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/nonownedwnd.h

@@ -30,14 +30,15 @@ public:
 
 #if defined(__WXDFB__)
     #include "wx/dfb/nonownedwnd.h"
+#elif defined(__WXGTK__)
+    #include "wx/gtk/nonownedwnd.h"
 #elif defined(__WXMAC__)
     #include "wx/osx/nonownedwnd.h"
 #elif defined(__WXMSW__)
     #include "wx/msw/nonownedwnd.h"
 #else
     // No special class needed in other ports, they can derive both wxTLW and
-    // wxPopupWindow directly from wxWindow and don't implement SetShape() (at
-    // least at this level, wxGTK does do it in wxTLW).
+    // wxPopupWindow directly from wxWindow and don't implement SetShape().
     class wxNonOwnedWindow : public wxNonOwnedWindowBase
     {
     };