Better Queue management

[vixalien]

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
• [ ] 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

maybe for #30