fix: Improve accessibility of colour slider plugin

[gonfunko]

The basics

• I validated my changes

The details

Proposed Changes

This PR improves the accessibility of the HSV colour slider plugin, and fixes a few bugs as well. It:

• Fixes focusing the first slider when the dropdown is opened
• Allow comitting the chosen color with Enter/Space
• Adds support for canceling and reverting to the previously selected color with Escape
• Adds ARIA labels to each of the sliders
• Indicates that the field has a menu popup, rather than the inherited grid
• Uses the standard focus ring indicator for the field, sliders, and eyedropper button
• Fixes a bug that would cause an exception to be thrown when dismissing the eyedropper with Escape
• Generates a description of the chosen color. This generally works well for pleasant colors, but very desaturated/dark ones are perhaps not ideal. This also would ideally be localized, but this is a bit of placeholder that will hopefully be replaced with something better soon-ish