Refactor UPower backend

Purpose: prepare for per-monitor screen brightness control.

Changes:

BackendInterface

• Add an addition device argument to brightness related methods to let user specify what brightness control they want to work with
• Add addition methods to let use query brightness control type and device label associate with it.
• Replace BrightnessControlsList with list and map for easily lookup.

PowerDevilUPowerBackend

• Implement changes from BackendInterface.
• Copy backlight device detection code from BacklightHelper.
• Detect and support control of both backlight devices and DDCUtil devices.
• Add necessary codes to support multiple brightness devices.
• Cleanup.

Actions: Update to new BackendInterface