Status bar indicator for current saved track

[martin-braun]

Indicates already saved (liked) songs that are currently playing by displaying a different status bar icon. It requires to sink changes of the player manager's currentTrackIsSaved published-object to react within the AppDelegate and toggle the icon.

Addresses #2, although it's no heart icon, I'm not good with SVG paths, so I just show a Spotify-green note instead. Please consider improving.

[martin-braun]

Preview:

The note turns green when the current track is within your "Liked Songs" playlist. I didn't want it to be too distracting, so I choose to keep the circle itself original.

It will only pick up the change when the player state changes (started track), the pop over is opened or when it's toggled within the pop over itself, not when it's toggled in Spotify directly.

[Peter-Schorn]

The new icon that displays when a track is liked does not have sufficient contrast with a light menu bar. The green music note is too hard to make out.

[martin-braun]

@Peter-Schorn I picked the Spotify green and I wanted it to be not intrusive. I agree that it's too hard to read, though. Do you maybe have a suggestion how to improve it? I was thinking about drawing an outline to the note, but it would require to change the shape of the note, as the green part would get too tiny when not doing so.