Raspberry PI standby mode

As the Raspberry PI has no proper standby mode from which it can be woken up again, it's so far impossible to turn off the attached display automatically.

This change set allows the PI's video output to be powered down, and back on again (works for HDMI and SDTV).

Also, there's a generic and reusable IPowerSyscall subclass CPowerSyscallVirtualSleep that allows devices to implement VirtualSleep/VirtualWake methods. For that to work the PowerManager now gets a chance to process and suppress events from CApplication::OnKey().

The PI's specific power management subclass is just a minor specialization of CPowerSyscallVirtualSleep which powers the video interface down during virtual sleep, and back on for virtual wake.