CC-4109: Playlist Builder: Add a button to remove all the items in the current editing playlist
CC-4904: Library -> Sort by status doesn't work

- added 2 columns to cc_files (is_scheduled, is_playlist)
- split library status column into two columns (scheduled, playlist)
- is_scheduled gets updated when a track plays out, or when a file gets added/removed from/to a show
- is_playlist gets updated when a file gets added/removed from/to a playlist/block, when a playlist/block gets deleted, or when a playlist/block's contents is cleared