Make wxGTK code returning correct focus in popup menu presence more robust.

Make wxGTK code returning correct focus in popup menu presence more robust.

Replace the changes of r69938 with an alternative and simpler solution:
instead of playing games with gs_pendingFocus, which could break down if its
old value became invalid while the menu was shown as happened if one of the
menu commands resulted in this window being destroyed, just take the currently
shown popup menu into account in DoFindFocus() itself.

This should be safer as there is no danger of any dangling pointers here and
is also simpler and more obviously correct.

Closes #14103.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71313 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775