Remove featured image support from playlists. May be used again in the future to set playlist artwork or as a fallback.
Minor whitespace tweak.
Update the template loader library to break both foreach loops when a template file has been detected.
Remove extra text domain filters.
Refactor admin code and include player feature.

Encapsulated the admin code in a class.

Players are registered by a theme hooking into the 'cue_players' filter.
Themes still need to build the players and their behavior, but this
feature allows Cue playlists to serve as the source of their tracks.