Refactoring of ListViewControlBase/GridViewControlBase to get rid of items.

- Items have been replaced for Cocoa-style delegates instead, so an
existing data structure does not have to be recreated for binding to the
ListView/GridView.
- PlaylistListViewControl has been partially implemented and is a work
in progress.
- OSX: Added playlist view into MainWindow, added
SessionsPlaylistListView, added WIP custom cells for punch in
buttons/text boxes (especially for loops).