Add several new CEC commands that can be used over D-Bus
This MR adds four commands for managing CEC that can be used over D-Bus:
- sendKey: Sends a CEC keypress, like the ones we listen for, but back to the TV
- powerOnDevices: Powers on one or all CEC devices, based on the logical address passed
- standbyDevices: Stands by one or all CEC devices, based on the logical address passed
- makeActive: Makes our LibCEC adapter the active source for the TV