Support mute streaming avatar locally and/or globally

Just read the comments of this issue: https://github.com/AUTUpStageTeam/upstage2013/issues/54, so I just redid it.

Now, there is two new context menu items for a streaming avatar: Toggle Locally and Toggle Globally. If the performer ends up having the avatar mute globally, but not locally (the performer hear him/herself, but the audiences do not), a warning alert will popup to tell the performer to unmute globally.