applets/brightness: consistently allow setting screen brightness to 0
Screen brightness had been artificially capped to 1 to prevent people from accidentally turning off their screen.
However this makes a useful actual minimum brightness on many screens inaccessible, and was inconsistent in behavior between the slider and the scroll action.
Introduce a Plasmoid configuration to deliberately and consistently allow setting screen brightness to the actual minimum.
Also remove the internal configure action, because there is no brightness KCM, so the default configuration action is unneeded and confusing.
Edited by Natalie Clarius