Customize TrackTableBackgroundColorOpacity from skins

[uklotzde]

Based on PR #2538 Ready for review now after prerequisite PR has been merged.

As suggested by @ronso0 here: #2539 (comment)

I'm not sure about the name TrackColorBackgroundOpacity. As an alternative this parameter could also be applied to the background color of the cover image that will be introduced in #2524. I don't use track colors and this would better fit my use case. More generic name is now TrackTableBackgroundColorOpacity.

Naming rationale:

• it is applied to the track table
• it controls the alpha channel (= opacity) of the background color
• the background color might be chosen from either the track color or the cover art background/preview color.

Due to many dependencies the implementation required more changes than expected. If we ever need more customizable parameters then we should rethink the design. But for now this works and should be approachable.

[ronso0]

Works as expected. Thx!
For later on it'll be helpful to set the text color, as well, to allow daylight schemes/skins having much more opaque track colors and an individually contrasting track font.

[Be-ing]

LGTM thanks. I'll leave it to @ronso0 to adjust the values in another PR if desired.