Draft: Handle application specific 'enable' notfication setting

This makes the per app "Notifications" toggle in g-c-c work.

This needed some tweaks to the source_id used for the PhoshNotificationBanner which in turn requried us to keep the desktop_id around too.

I'll unwip after brushing up the tests and more testing (mostly cobbled this together during the GTK BoF)

/cc @zbrown would be great to hear if this matches what you proposed in #168

Signed-off-by: Guido Günther guido.gunther@puri.sm