fix(designer-ui): Allow color picker sliders to be accessed by keyboard

[ek68794998]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix (a11y)

• What is the current behavior? (You can also link to an open issue here)

Color picker hex field and palette buttons can be accessed by keyboard, but the two sliders cannot.

• What is the new behavior (if this is a feature change)?

All fields of the color picker can now be accessed by keyboard, and controlled using arrow keys (and optionally +shift to move a larger distance at once).

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Please Include Screenshots or Videos of the intended change:

[rllyy97]

This is really cool!