Delete MythUINotificationCenter when quitting applications.

Also add some safeguard, just in case some messages are received while we're currently deleting the notification center
Handle PNG coverarts

Make it so that we should handle more type should they ever be sent by iTunes
Implement progress bar in UIMythNotificationScreen

Create new MythNotification: Playback that contains playback progress and Media that includes an image and playback progress.
Only display UI elements that are actually in use.

Redefine MythNotification::Update type: only fields changed are actually refreshed; this prevent redrawing all fields all the time
Use new progress bar in MythUINotificationCenter to display duration and position of song currently playing