Make color slider disabled state consistent with disabled color area

[bjarnef]

Description

When <uui-color-area> has disabled attribute it has some opacity and not-allowed cursor.
The <uui-color-slider> did not, so it wasn't clear the slider was disabled besides it wasn't possible to drag handle.

This PR makes this more consistent.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

How to test?

Screenshots (if appropriate)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

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[bjarnef]

We may need to have a inner element as cursor and pointer-events doesn't work on same element (cursor need to be on parent and pointer-events on child).

However <uui-color-slider> this also mean the label has not-allowed style. Not sure we want that?

[iOvergaard]

I think it's fine as it is, but it comes down to taste. I will merge this and we can consider extending it as you suggest if it doesn't feel right after being released.