MPRIS is tied with PlayerToolbar because the thumbnail updating logic
takes place in the CoverStack from PlayerToolbar.
In fact, on every song change the following thing happens:
* PlayerToolbar looks up for the song's thumbnail (via its
  cover_stack) and emits a "thumbnail-updated" signal once the
  operation is finished.
* MPRIS sends the song's metadata without the thumbnail url (it has
  not been updated yet). A bit later, once the "thumbnail-updated"
  signal is received, the metadata are sent again with the correct
  thumbnail url.
This can result in a glitter on some MPRIS clients: the client is
updated with an empty thumbnail, and then, very quickly, with the
correct one.

Fix that issue by factoring out the thumbnail cache lookup and
directly looking up the song thumbnail from MPRIS. The disadvantage of
this solution is that the lookup_art_file_from_cache function is
called two times on a song change. One call from PlayerToolbar and the
other one from mpris. However, this function call is less costy than
sending two times the song's metadata.

Related: #43, #172
Closes: #101