Cover player improvements

[digimezzo]

Reported by @yurisuika in #660.

I like the mini player, but I have a couple suggestions!

I think it would appear best if the title/artist, window controls, and gradients transitioned out of opacity when the window is not in focus. This would provide a much cleaner look for such a mode that is "focused" on the artwork.

As well, the song title is a bit large for such a small window. Even relatively short titles of several words are truncated at this scale. This is also an issue with Showcase mode to the same extent. I am sure it looks great for song titles that are a single word, but that is a very limited use case.

To summarize:

• Fade out controls and title/artist and show the mon mouse move (like now playing)
• Decrease song title
• Improve fade for light covers