mpris: Synchronize metadata emission on song change

When a song changes, two signals are emitted: "current-song-changed" and "thumbnail-updated". Most of the time, "current-song-changed" will arrive first. So, the metadata property will be sent with an empty artUrl. Then, the thumbnail_updated method will send again the metadata with the correct artUrl. By adding a timeout, it waits for the arrival of the second signal and prevents to send two different signals.

Related: #43 (closed), #101 (closed)