* Support layer suspension

Add support for a message which notifies the endpoint whether or not it
is selected (meaning its HD stream is in use).  If it is not
selected and enableLayerSuspension is set to true then it will impose a
bandwidth limit in the SDP to suspend sending the higher layers.

* only add the IS_SELECTED_CHANGED listener in JingleSessionPC if layer
suspension is enabled

this prevents doing a local o/a when we don't need it

They are really verbose, and add no real value. In addition, log the channel
type once opened.

Some browsers may not support data channels (hi there, Edge!) and sometimes it
may not be desirable for other reasons.