applets/brightness: Check if KeyboardBrightness action is available
Besides reacting to changes in the Solid.PowerManagement DBus service,
we also need to track changes in the KeyboardBrightnessControl action
(which translates into a specific DBus path) before trying to call or
connect to that path and fail.
This doesn't fix the race conditions on plasma login, but allows the applet to react live to changes in the action list.
The first commit adds the supportedActionsChanged D-Bus signal to the
daemon so the applet plugin can connect to it and get notified
Edited by Ismael Asensio