conference: add secondary input

bin/dbus/cx.ring.Ring.CallManager.xml

bin/dbus/dbuscallmanager.cpp

bin/dbus/dbuscallmanager.h

src/client/callmanager.cpp

src/client/videomanager.cpp

src/conference.cpp

src/conference.h

src/dring/callmanager_interface.h

src/media/video/video_mixer.cpp

src/media/video/video_mixer.h