[Voice Update] LLPanelActiveSpeakers to LLParticipantList

Minor updates to code, mostly just a migration with some v3 stuff commented, since we don't have LLAvatarList and must use buttons since scrolllist menus don't actually exist nicely yet.
Added toggleMuteVoice and isVoiceMuted to LLAvatarActions
Added a minor touch to the voice volume slider, it'll now offer changing your own volume while in an active voice session.. nice to not drop out of an import voice session just to fix yourself.

Also, fix a bug with SpeakerMgr's not flagging speakers for removal properly, causing external speakers and speakers managed by LLActiveSpeakerMgr to never actually get removed.
[Voice Update] Fix the current voice chat button

If the current voice chat's floater is undocked, grab it by session id so that it's not labeled blank
When the button is clicked, open the current voice chat floater, not the communications floater, this is the intended action.. how it should have acted in the first place.