Per-monitor screen brightness

Currently powerdevil and the battery/brightness applet only knows one screen brightness.

This of course falls on the nose when we have more than one monitor with working brightness control.

UI-wise we probably want a slider per controllable monitor in the applet, with some indication which monitor is being controlled. What should happen when pressing the brightness keys on the keyboard is a bit unclear.

Implementation-wise the complexity is that there's two ways to control the brightness: Via sysfs (BacklightHelper in powerdevil) and via ddcutil. sysfs is mostly relevant for laptop displays whereas ddcutil is mostly relevant for external monitors. Are there screens that support both?

What we need to do is:

1. In powerdevil: Detect all controllable screens by aggregating sysfs and ddcutil results. We also need some sort of identifier (connector name? EDID?) and user-visible label
2. Expose that to the applet (via DBus?)
3. In the applet: Show a slider for each thing
4. When the slider changes write back to Powerdevil
5. In powerdevil: Write the change to the display via the right mechanism