mpris: Limit Seeked signal emissions

According to MPRIS specifications, the Seeked signal should only be
sent when the position has changed in a way that is inconsistant with
the current playing state (for example, when the song position has
been changed by SmoothScale).

This issue is fixed by storing the song position every time a request
is made and emit a Seeked signal if the difference between the new and
the previous position is greater than 2 seconds (It seems like a good
default value).

Related: #43
5 jobs for wip/jfelder/mpris-seek in 12 minutes and 15 seconds (queued for 3 seconds)