[0.3] Some small Playlist/Tray icon tweaks

[Dan O'Reilly]

Attached is a patch that
1) Adds the currently playing track to the tray icon tooltip.
2) Adds a key_release_event listener to xlgui/playlist.py so that the pressing the delete key will remove tracks from the playlist.
3) Removes the call that temporarily set the DragTreeView model to a blank model when reloading tracks. As far as I can it isn't necessary (just clearing the existing model seems to be enough) and its inclusion causes the scrollbar position to reset to the beginning of the playlist when a track is removed, which can be a little bit annoying.

[reacocard (reacocard)]

Committed r1719