player: Use its own simplified model

Player keeps an internal reference to the currently visible view, wich
can be different from the currently played model. The main idea is to
isolate player model. when set_playlist method from player is called, a
simple copy is done. Therefore, when view changes, player model is not
updated. This new model needs to stay synchronized by calling add_song
or remove_song appropriately.
This is not a fix. Just a workaround before player.py rewrite.
Add an enum for player model fields.
Remove 'song-position-changed' signal from playlists as it is not
necessary anymore.

Closes: #136
3 jobs for wip/jfelder/simplify-player-model in 7 minutes and 1 second (queued for 6 seconds)
Status Job ID Name Coverage
  Check
passed #11553
pep8

00:02:20

passed #11552
pyflakes

00:02:19

 
  Build
passed #11554
build

00:04:40