[nanoleaf] More color for less network calls (#13893)

[nanoleaf] More color for less network calls (#13893)

* [nanoleaf] More color for less network calls

This is a refactoring that moves the "get panel color" out of the
panel handler and into a separate class, with callbacks.

This makes us do only one REST call to get colors instead of one per
panel that is a thing. It also lets us retrieve colors for all panels -
 also those that doesn't have a thing in OpenHAB,

While testing this out, I found a bug where solid colors set in the app
wasn't reflected in neither the controller nor panel channels, and that
should also be fixed now.

Signed-off-by: Jørgen Austvik <jaustvik@acm.org>