WColorPicker: always show custom color button

[Be-ing]

There's no need to hide the custom color button. It is useful anywhere a color can be picked. Following discussion from #2557.

This reveals a bug with the styling. Sometimes the custom color button shows a black background. Steps to reproduce:

1. set a color of a hotcue from the palette (not the custom color button)
2. change the color of that hotcue using the custom color button to select a color outside the palette
3. open the cue menu again. The custom color button only shows the gradient while the mouse is held down

Any hints what might be going wrong here would be appreciated. The styling is done in res/skins/default.qss

I propose to target this for 2.3. There is no need to block the beta release for this.

[Holzhaus]

We could make that configurable if you want, but wouldn't want to spawn a full-blown QColorDialog during a set because I accidently clicked the wrong button. Also, why not add the additional colors to the palette but configure the auto-assignment in a way that only the first 8 are used?

[Holzhaus]

. Sometimes the custom color button shows a black background.

I maybe we need to add styles for the :hover case, too? I don't really know. Maybe a Pipette icon would be bettern than the gradient anyway. Since the custom color button isn't checkable anyway, this wouldn't interfere with a checkmark icon anyway. @ronso0 can you look into that?

[daschuer]

IMHO we should not add a custom color button to the palette.

This encourages the user to use any color, this is a tempting solution for a problem he might has creating other problems.

I think setting up pallets an custom colors should not be done in the heat of the night.

We should not clutter the color picker.

@Be-ing: what is the underlying use case you like to solve?

[ronso0]

I'm also not sure which issue this is supposed to solve.
Do you want to pick a unique color for special markers?
For users that are missing a color from the palette a ? (icon?) with a tooltip mentioning Pref > Colors would be more helpful and make the palette editor more discoverable.

[daschuer]

Can this b closed?

[Be-ing]

Sure, I don't care much either way.