notification: Pass a tuple to variant instead of list

The serialized data for the media art should be a tuple, not a list.
This makes the update of the media art to fail if it is not found.

https://bugzilla.gnome.org/show_bug.cgi?id=729100
albumArtCache: Don't return path if doesn't exists

Since an invalid path is used by notification to know if the media art
is found, return None if it doesn't exists.

https://bugzilla.gnome.org/show_bug.cgi?id=729102
albumArtCache: Fix full retrieve of song items

Check first if the item is a song before retrieving the item's thumbnail
with full options. The old code always assumes that the item is an
album. Then use a different query to retrieve the album if it is a song.

https://bugzilla.gnome.org/show_bug.cgi?id=729102
notification: Don't lookup thumbnail

We could just use the thumbnail found by the player by connecting to
thumbnail-updated instead of looking it up again.

Also, serialize the default thumbnail on init which takes some seconds
to finish.

This removes the delay on the update of the notification when the media
art is not found.

https://bugzilla.gnome.org/show_bug.cgi?id=729102