Playlist UI bug fixes.

- Allow playlist to load even if it uses events which no longer exist.
  This needs further work, but is better than it was before.  We need
  to sanity check the playlist when it is loaded so we can alert the
  user if either the media, sequence, or event file does not exist.

- When adding a new playlist, allow hitting enter to save the new
  playlist name, just as it would if we were using a form instead
  of an onClick event on the 'Add' button.

- Whenever we populate the Media, Sequence, and Event dropdowns in
  the Playlist editor, call PlaylistTypeChanged() so that we display
  the right dropdowns based on the current playlist entry type selected.

Fixes #80.
Fixes #81.