notifications: Add better debug for saved apps

So we can see the apps for which there already are saved settings.
notifications: Fix duplicates in application list

When saving applications in the "application-children" list, gnome-shell
will lower-case the whole canonical application ID in _canonicalizeId().

Match _canonicalizeId()'s behaviour, and lower-case the canonical
application ID so we don't try to load "org-gnome-Software" from
GNOME-Software's desktop file in addition to the already saved
"org-gnome-software".

https://bugzilla.gnome.org/show_bug.cgi?id=729447