Playback window cleanup and Quick Look for album art.

- Use black-tinted NSLevelIndicator rather than NSProgressIndicator for track playback - eliminates distracting animation.

- Use NSButton with border on mouse-over rather than NSImageView for album art - cleaner and permits clicking, which summons Quick Look.

- Make control margins consistent.
Clean up progress update timer.

Window was nil when -[PlaybackController init] ran, so NSWindowWillCloseNotification/NSWindowDidBecomeKeyNotification were firing for other windows, causing progress updates to be inappropriately suspended.  Use -awakeFromNib instead so the window is set up.

No need to initialize instance variables; they're nil by default.

No need to update progress more often than once per second.

Don't run timer when playback is paused or Hermes is hidden.