Add Hotcue Button colors to skins

[Holzhaus]

This gives skins the ability to show Hotcue Button colors and adds support for that feature to the Deere skin. I used to colors from PR #2031 (and just added 0x20 to the dominant RGB channels for hover colors).

This resolves Launchpad Bug lp1829884 (but only partially, since I only adapted the Deere skin).

Here's a screenshot:

[Holzhaus]

I also added support for colored buttons to the other skins (except LateNight).

[Holzhaus]

@uklotzde IMHO this PR is ready. The only skin missing is the LateNight skin, but I have no idea how to add support to it, because it uses SVG files that contain gradients. In any case, colored hotcue support could be added to LateNight at a later date.

[ferranlala]

Good job, this PR looks good and it's a long awaited feature!

Can you tweak the default hotcue mark color on Tango and Deere so it matches the default color of the hotcue button?

[Holzhaus]

@ferranpujolcamins Done.

[ferranlala]

LGTM

[uklotzde]

LGTM. Very helpful, especially in conjunction with the auto-assignment of cue colors!

[ronso0]

LateNight is addressed in #2205

[ronso0]

@Holzhaus Could you explain which hotcue state [displayValue="2"] should address?
Different css colors for "1" and "2" don't yield any visible results, color from "1" is used for enabled and pressed buttons.
I'm trying to style both the hotcue_X_enabled and the _activate (set or play from hotcue) state.

[ronso0]

@Holzhaus
In Shade (and only there) hotcue button #1 (and only this) never get's painted with the assigned color. Do you have any idea why this happpens?
The qss is correct, it doesn't matter at which position the marker is set, it doesn't matter if button#1 is the first hoctue template that's loaded...it's just weird.

[Holzhaus]

If the qss is okay, maybe the widget is not re-polished? But that shouldn't be skin-dependent...