Added wxWindow::GTKHandleRealized() virtual method to wxGTK.

Added wxWindow::GTKHandleRealized() virtual method to wxGTK.

This allows to easily do something in the derived classes when the widget is
realized, without having to deal with GTK+ signals. In particular, get rid of
another "realize" signal handler in wxTopLevelWindow and simply override this
virtual method there.

It also incidentally makes the callback code simpler as the window doesn't
need to be constantly dereferenced.

This shouldn't result in any changes to behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69390 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775