Implement SetShape() for wxPopupWindow in wxGTK.

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