shellDBus: Fix FocusApp()

shellDBus: Fix FocusApp()

The appDisplay used to be accessible via the viewSelector, but
that is gone now. Commit c09c070b15 adjusted the code, but the
replacement uses a Main.overview.appDisplay accessor that never
existed, whoops.

Fix this by exposing a new selectApp() method directly on the
overview, so we don't have to shuffle the appDisplay through
controls → overviewActor → overview.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1891>