Rename desktop entry to all-lower-case

GNOME Shell appears to normalize desktop entry IDs (basenames)
to all-lower-case. I have no idea whether this is standard,
but if we consistently use lower-case we won't have to
worry about it.
Set X-GNOME-UsesNotifications so we appear in gnome-control-center
Merge pull request #46 from smcv/notifications

Improved notifications in GNOME Shell 3.8