SignalLevelAgent hierarchy
==========================

Service		unique name
Interface	net.connman.iwd.SignalLevelAgent
Object path	freely definable

Methods		void Release(object device)

			This method gets called when the service daemon
			unregisters the agent. An agent can use it to do
			cleanup tasks. There is no need to unregister the
			agent, because when this method gets called it has
			already been unregistered.

		string SignalLevelChanged(object device, uint8 level)

			This method gets called when the signal strength
			measurement for the device's connected network
			changes enough to go from one level to another out
			of the N ranges defined by the array of (N-1)
			threshold values passed to
			net.connman.iwd.Device.RegisterSignalLevelAgent
			(see device-api.txt.)  The level parameter is in
			the range from 0 to N, 0 being the strongest
			signal or above the first threshold value in the
			array, and N being the weakest and below the
			last threshold value.  For example if
			RegisterSignalLevelAgent was called with the
			array [-40, -50, -60], the 'level' parameter of
			0 would mean signal is received at -40 or more dBm
			and 3 would mean below -60 dBm and might correspond
			to 1 out of 4 bars on a UI signal meter.