The GStreamer clock stops ticking as soon as a Grilo writeback is
triggered. This does not pose as an immediate problem, as it is currently
only relied upon for scrobbling, triggering Tracker writeback and passing
on the clock-tick. However, the former two are one-time events per song and
the latter is also provided by other means.

It will be a problem as soon as the pipeline and clock do not reset between
songs when gapless playback is introduced. The exact core of the problem is
unknown, but might be related to Grilo async handling: grilo#81.

Calling the Grilo writeback function in an idle_add seems to resolve the
issue until it is fixed upstream.