gtkmain: Don't process WM events if there's a grab on another toplevel

gtkmain: Don't process WM events if there's a grab on another toplevel

This is implicitly done for us in the case of grabs on windows from other
groups, but we must perform this check explicitly for grabs with
owner_events=True on windows from the same group, in that case the window
would handle the events as if there was no grab.

https://bugzilla.gnome.org/show_bug.cgi?id=752327