window-tracker: Fix minor style issue
window-tracker: Also listen for changes in the gtk-application-id

Wayland applications don't tend to have very useful WM Class properties,
as GTK+ isn't very good at picking an appropriate application ID. While
we should likely fix GTK+ to pick a better app ID, we do have the
existing gtk_shell for more accurate information. The only problem is
that the gtk_surface is set after the MetaWindow is constructed, and
we're not listening for changes on the GTK+ application ID.

Listen to changes on the GTK+ application ID to fix app tracking for
most GTK+ applications under Wayland.