js: Change main.pushModal to return the Clutter.Grab handle

js: Change main.pushModal to return the Clutter.Grab handle

All callers have been updated to keep this handle to identify their
own grab.

Also, optionally use the windowing state to determine whether
the grab is suitable for the specific uses. This removes the need
to trying to grab twice in the places where we settle for a keyboard
grab.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2045>