Better Queue management

Better queue management includes following:

[x] Add to queue for songs and playlists
[ ] Fetch more queue items for huge playlists
[ ] Sometimes, when changing automatic queues, the track is duplicated (possibly fixed)
[ ] Maybe even allow the Queue to implement the Gio.ListModel interface directly
[x] Show a context menu for queue items, allowing to like them, etc
[ ] Allow rearranging queue items
[ ] Show the autoplay slider as a GtkSectionTitle
[ ] Allow disabling autoplay
[ ] add menu to now playing menu to allow adding current track to library (and maybe add queue, sleep timer, etc...)

maybe for #30

vixalien / muzika