wayland: add support for controlling brightness devices exposed by powerdevil
This way, KWin can set the brightness on internal panels or external monitors with DDC/CI support, without being exposed to the mess that is actually setting the brightness that way.
Depends on libraries/plasma-wayland-protocols!78. Together with libkscreen!186 and powerdevil!374, the brightness slider still works, just as it does with everything on master, but it's routed through KWin.