added video option to prioritize a user's webcam when they start talking

bigbluebutton-client/resources/config.xml.template

.../bigbluebutton/modules/users/views/MediaItemRenderer.mxml

...igbluebutton/modules/videoconf/business/VideoWindowItf.as

...igbluebutton/modules/videoconf/events/UserTalkingEvent.as

...bigbluebutton/modules/videoconf/model/VideoConfOptions.as

...g/bigbluebutton/modules/videoconf/views/AvatarWindow.mxml

.../bigbluebutton/modules/videoconf/views/PublishWindow.mxml

...rg/bigbluebutton/modules/videoconf/views/VideoWindow.mxml

.../org/bigbluebutton/modules/videodock/views/VideoDock.mxml