Don't set invoking window recursively in wxGTK wxWindow::PopupMenu().

Don't set invoking window recursively in wxGTK wxWindow::PopupMenu().

Setting the invoking window for all submenus is unnecessary as
wxMenu::GetWindow() recurses upwards anyhow and results in assert failures
after recent menu code changes.

Simply don't do this.

OTOH do reset the invoking window to NULL after the menu is dismissed to avoid
storing a dangling pointer in the menu.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64142 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775