Apply more saturated color scheme to layers and highlight more distinct

[MrStevns]

In correlation to #1308 which was made to get more distinct highlight, I've come up with this change. The highlight is always based on the color behind.

[davidlamhauge]

I can fully support this elegant solution, for a more visible highlight for the active layer.
I'm still puzzled and confused over the fact, that I was more than a year to see that there indeed was a faint white shadow on the active layer. We really need a more visible highlight for the active layer.
I might even be able to review it, if we can agree on the change 😉

[davidlamhauge]

This PR was initiated, since I repeatedly drew on the wrong layer, since the layer highlight was too vague for me to see.
@Jose-Moreno and I have approved of the more saturated colors. If the rest of the team agree too, then let's merge it into the master.
LGTM!

[scribblemaniac]

Looks good to me. Honestly I've never had an issue with seeing the highlight before, it's what I always use to see what layer I'm on, but this does make it much more visible. I personally think that we should also add a gradient to the label section when not selected. Something like this for example:

That is a gradient from (255, 2555, 255, 63) to (0, 0, 0, 0) with the same code as paintSelection.
The track could also have the gradient, but it's not as noticable unless you've got a bunch of the same layer type next to each other:

[MrStevns]

I've applied gradient to tracks and labels that are not selected, now there's definitely no doubt that a track is selected.