There is now a `on_color_changed` callback that clients can optionally
hook into to receive real-time updates while the user is picking a
color. If the user hits Cancel, the callback gets called once more with
the color passed in while constructing `ColorPicker`. If the user hits
OK, the same happens with the currently selected color instead.
Programs therefore can perform all their updates with this callback, and
only care about `ExecResult` if they want to make a decision, like if we
should write the result to `ConfigServer`, for example.
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