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, #101
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