Feature Request: Add eyepicker tool to "Pick Color"

It would sometimes be nice if I could just pick a color from an ongoing render, or the color of a block in the chunky window.

No source files, tests, or entry points are identified. Start by locating the Pick Color action and the chunky window, then determine how an ongoing render or block color can be selected; done means the requested eyepicker behavior works in both contexts.

Written by the indexing model from the issue text.