applets/brightness: add control for syncing device leds with accent color
Screencast: https://imgur.com/a/sd9Lqz9
The control is only shown when there are any RGB capable LEDs on the device and the backend is available, i.e. kdeplasma-addons is installed and the kded background service is not disabled.
Depends on backend: kdeplasma-addons!560 (merged)
Edited by Natalie Clarius