application: Fix close command for MPRIS

The parameters of quit() method changed but was not updated in MPRIS.
Set the new parameters as optional.

https://bugzilla.gnome.org/show_bug.cgi?id=707632
player: Add new signal for thumbnail update

We no longer resolves the thumbnail path when populating the views, so
get_thumbnail() is unusable on all media on first play.

This opens a way to tell when the thumbnail becomes available.

https://bugzilla.gnome.org/show_bug.cgi?id=708102
albumArtCache: Save thumbnail path as URI

https://bugzilla.gnome.org/show_bug.cgi?id=708102
mpris: Send PropertiesChanged signal when thumbnail is updated

Resends the metadata when the thumbnail is updated. We could send just
the mpris:artUrl key, but some clients ignore metadata that contains
only one key (some music players send a dummy value on stop), so send
all metadata to be safe.

https://bugzilla.gnome.org/show_bug.cgi?id=708102
mpris: Seek to start when a song is loaded and in repeat song mode

This makes sure that the current position will be reset when replaying
the song. This seems like it doesn't follow the MPRIS specifications (no
seeking when the media changes), but other media players do this too, so
do it anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=703303
player: Don't emit playing-changed when the bus state changes

This causes a crash in notification when the current track gets hidden
on filtering, but when the bus state changes, it doesn't mean that the
current track changed.

https://bugzilla.gnome.org/show_bug.cgi?id=708116