ColorPicker add ability to set alpha channel

This was a request from gitter chat. Logging it here so it does not get forgotten.

Currently the new ColorPicker control does not allow for setting transparency (alpha channel). We should consider UI options that allow for controlling this.

Start by locating the new ColorPicker control and reviewing how it currently represents and edits color values. Decide on a UI option for controlling the alpha channel, then verify that users can set transparency through the control and that the behavior is covered by the project’s existing checks.

Written by the indexing model from the issue text.