plugins: enable audio/video plugins when hosting a conference

src/conference.cpp

src/conference.h

src/media/video/video_mixer.h

src/media/video/video_rtp_session.cpp

src/media/video/video_rtp_session.h

src/observer.h

src/plugin/callservicesmanager.cpp

src/plugin/chatservicesmanager.cpp

src/plugin/pluginmanager.cpp

src/plugin/pluginmanager.h

src/sip/sipcall.cpp

src/sip/sipcall.h