Check for GTK window parent, not wx one, when reparenting.

Check for GTK window parent, not wx one, when reparenting.

A notebook page removed from its notebook can have a non-NULL wx parent but
not have any parent at GTK level any longer, so test for the window parent at
GTK level before removing the widget from it, otherwise we were getting GTK
asserts when trying to remove a window from its no-more-parent.

See #13305.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68213 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775