improve notifications in GNOME Shell 3.8

GNOME Shell 3.8 lets you configure notification sources (identified by the basename of their .desktop file, normalized to lower-case for some reason) to show more or fewer details.

For instance, notifications' text doesn't appear in the lock screen by default, but mpDris is one of the apps for which it's reasonable for the notification ("now playing...") to appear on the lock screen; combined with multimedia keys now working in the lock screen, this gives you basic control over playback without unlocking the screen (particularly useful if, like me, you use mpd-dynamic to have a random playlist that never ends).

This branch hooks up mpDris as a notification source.