Fix xdg_popup leak

xdg_popup_destroy() only logged "Not implemented" instead of calling
wl_resource_destroy() like every other role object's destroy handler,
and there was no xdg_popup_destroy_resource() override to free the C++
object either.

Make xdg_popup_destroy() actually destroy the resource, and add a
xdg_popup_destroy_resource() override that deletes the C++ object,
matching xdg_toplevel/xdg_surface's existing pattern.

Pick-to: 6.8 6.11 6.12
Task-number: QTBUG-148611
Change-Id: Ic35aea92e9423d2956af213b1586429efde8eccb
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>