optionally listen for upower suspend/resume events to stop/start all receiver listeners
disable poll ticking

It was necessary to periodically check if the peripherals are still
online -- suspend/resume may cause Solaar to (wrongfully) remember the
status of devices after a resume.

Now that is handled by the optional upower module -- the hard way -- by
restarting all listeners on resume.
some extra asserts on hid++ 1.0 register r/w
properly handle tree re-selection when removing a receiver
don't use proxies for receiver/device objects

they should be unnecessary now that their status is properly tracked
initial support for performance mx leds

The leds light up when we get updates about the battery level.
Right now they are not (programatically) turned off.