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.h

@@ -92,7 +92,7 @@ struct LLGroupData
 
 //
 
-class LLAgent : public LLObservable
+class LLAgent : public LLOldEvents::LLObservable
 {
 	LOG_CLASS(LLAgent);