Add playlist parser

[Rafostar]

We should have an internal playlist parser. Similar functionality to Totem playlist parser, but done differently.

Some early ideas:

• It should be in the form of GStreamer element that messages app with contents while also forwards first URI from it for playback.
• Media items have subitems list model
• Subitems can be found during either playback or via discoverer
• An advance subitem functionality in API (different from queue next/previous)
• Integration with Clapper enhancers (e.g. support YT playlists)
• UI should show subitems from item on click
• ClapperGtk next/previous buttons should advance subitems if playlist item is selected
• Reworked queue progression logic to move/shuffle along subitems
• Opening a directory should be treated the same as item that is a playlist
• Custom .claps file reading should be moved to this new logic too

Related: #197 #476 #504