panel: Only update appMenu icon when the app actually changed

panel: Only update appMenu icon when the app actually changed

Right now we always recreate the icon of the appMenu when calling
_sync(). This will relayout the panel everytime we open the overview,
because we call _sync() in that case.

We can easily avoid that by only recreating the icon actor in case the
app that's opened actually changes. This gets us close to doing no more
relayouts of the panel when opening the overview.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1733>