Move AIAlert* into namespace AIAlert and rename classes.

Instead of having several AI* classes, it turned out to be easier to
have a namespace: this allows me to define the classes in llcommon but
add (new) 'add' and 'add_modal' functions to 'AIAlert' in llui.
This is needed to avoid a collision with the 'add' functions in
LLNotificationsUtil.

The new add/add_modal makes it a lot easier to just show a caught
alert, prepending or appending new text: it turns out that that is
way more common then a re-throw.

Adjusted code as appropriate.

indra/llui/llnotifications.h

@@ -108,7 +108,7 @@
 #include "llnotificationptr.h"
 #include "llnotificationcontext.h"
 
-class AIAlert;
+namespace AIAlert { class Error; }
 
 typedef enum e_notification_priority
 {
@@ -739,7 +739,7 @@ public:
 						const LLSD& substitutions,
 						const LLSD& payload,
 						LLNotificationFunctorRegistry::ResponseFunctor functor);
-	LLNotificationPtr add(AIAlert const& alert, unsigned int suppress_mask);
+	LLNotificationPtr add(AIAlert::Error const& error, int type, unsigned int suppress_mask);
 	LLNotificationPtr add(const LLNotification::Params& p);
 
 	void forceResponse(const LLNotification::Params& params, S32 option);