LLViewerRegion WIP: Added all headers that are dragged in.

Most in particular llevents.h, which comes along with
the demand that the old events in llevent.h are put
in a namespace LLOldEvents. Made all changes necessary
to compile the rest of the code again (without changing
the actual code: it's still using the old events).

This patch also removes LLStopWhenHandled and LLStandardSignal
from indra/llui/llnotifications.h because those are
moved to llevents.h. That seems to be the only change
to indra/llui/llnotifications.h that isn't floater related,
so I left the rest of that file alone.

indra/newview/llagent.cpp

@@ -2818,7 +2818,7 @@ void update_group_floaters(const LLUUID& group_id)
 		gIMMgr->refresh();
 	}
 
-	gAgent.fireEvent(new LLEvent(&gAgent, "new group"), "");
+	gAgent.fireEvent(new LLOldEvents::LLEvent(&gAgent, "new group"), "");
 }
 
 // static