Fixes echo problem with native simulcast when the local participant is selected.
Minor simulcast code cleanup.
Renames simulcast related classes.
Splits the simulcast management in two parts, a receiver and a sender.

Splits the simulcast management in two classes, a SimulcastReceiver and a SimulcastSender. A SimulcastManager class manages the receiver and the sender. The SimulcastSender class specializes into the NativeSimulcastSender, SimpleSimulcastSender and NoSimulcastSender classes.
Attempts to prevent blackness when switching simulcast streams.