gtkmain: Ignore grab for events in child popovers

gtkmain: Ignore grab for events in child popovers

Popovers may be spawn when there's GTK+ grabs somewhere else (eg.
text selection popover/handles in an entry in a modal popover). When
this happens, events go to the grab widget (in this case the modal
popover) and are effectively ignored by the event widget, even though
it's can be conceptually a child of the grab widget.

To get away with this, tweak a bit gtk_main_do_event(), so events going
to popovers that are related to grab_widget or a child of it are received,
as it would happen with regular children of grab_widget.

https://bugzilla.gnome.org/show_bug.cgi?id=750993