PlaylistTrackCellView: remove old unused drawRect
Add option for disabling placeholder album art in library preferences

- Add new checkbox in library preferences tab for enabling/disabling
  showing placeholder album covers
  - This is bound to UserDefaults.libraryViewShowPlaceholderAlbumCovers
- Make UserDefaults.libraryViewShowPlaceholderAlbumCovers default to YES
  using defaultPrefs.plist
- Return nil (blank image) in [LibraryViewAlbumCell placeholderImage] if
  UserDefaults.libraryViewShowPlaceholderAlbumCovers is NO
- Also set up an observer on UserDefaults.libraryViewShowPlaceholderAlbumCovers
  - This is to watch for changes in the preference and to make them
    occur immediately
  - Basically if there is no cover, it re-calls placeholderImage which
    takes into account the new preference value

- For issue #50
PlaylistTrackCellView: fix autoresizing mask

- Now right margin is fixed, which is what we want
- Also seems to fix a text rendering glitch where sub-pixel AA would be
  randomly turned off during heavy scrolling.