overview: Don't block on wayland popovers

We don't allow entering the overview when there's an ongoing grab
operation, as they are generally incompatible with ClutterGrab.

The exception are wayland popups which are expected to work, and
don't conflict with ClutterGrabs.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6092

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2549>
(cherry picked from commit a4e2d708)