mpris: Fix object path for track ids

Follow the MPRIS2 spec regarding TrackList track id's and encode the
Grilo song id so it cannot contain invalid characters.

Related: #100
3 jobs for wip/mschraal/mpris-object-path in 20 minutes and 26 seconds (queued for 44 minutes and 31 seconds)
Status Job ID Name Coverage
  Check
passed #5637
pep8

00:04:30

passed #5636
pyflakes

00:04:45

 
  Build
passed #5638
build

00:11:10