customize urgency behaviour

[shazow]

libnotify supports low/normal/critical urgency, we want to be able to mimic what android does for "silent" notifications vs normal.

some example scenarios that would be nice to support:

• for critical notifications, dim the rest of the screen for 1s and highlight the notification.
• for normal notifications, have a "slide in" animation and expire after 5s (slide out)
• low urgency, skip the animation, just fade in and fade out over 3s.