Added a Playlist class

The starred playlist functionality uses the new Playlist class. Playlist
still offers the native sp_playlist pointer via its interface but isn't
meant to stay like that.
libspokify file naming convention

Public files use CamelCase and internal files user alllowercase.
Session::playlistContainer returns a pointer instead of a reference

The returned value is NULL when the user is not logged in. When the
logging in happens MainWindow passes the current playlist container to
PlaylistView.

SessionMaster destroys the playlist container when the logging out
happens and creates a new one after login.
PlaylistContainer::playlists returns Playlist objects instead of native objects
Playlist can be queried for its name
PlaylistModel contains Playlist pointers instead of sp_playlist pointers
Playlists can be renamed

PlaylistView uses the libspokify interface as much as possible.
Added Search, SearchResults and Track to libspokify

The search functionality is now hidden in the library. When search
results become available a signal is emitted.

Quite a big change but couldn't make it smaller than this :(
Replaced sp_track with libspokify::Track in MimeData and TrackModel

The changes propagated to a lot of places. Added tracks() method to
Playlist among others.