mpris: Do not send the Stopped signal when a song ends

MPRIS specifications are not explicit whether the "Stopped" signal
should be sent at the end of a song. It seems logical not to emit this
signal if the playlist is not finished because a new song is gonna
start. Besides, this is the behavior of all the usual audio players.

This is fixed by returning the "Playing" PlaybackStatus if the player
is in the LOADING (which happens at the end of all songs) or PLAYING
state.

Related: #282
2 jobs for master in 14 minutes and 23 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #311900
flake8

00:01:46

 
  Build Flatpak
passed #311901
flatpak

00:12:37