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/llcommon/aifile.cpp

@@ -55,7 +55,7 @@ AIFile::~AIFile()
 	int errn = errno; \
 	std::ostringstream error; \
 	error << LLFile::strerr(errn) << " (" << errn << ')'; \
-	THROW_MALERT_CLASS(AIAlertCode, errn, __VA_ARGS__ ("[ERROR]", error.str())); \
+	THROW_MALERT_CLASS(AIAlert::ErrorCode, errn, __VA_ARGS__ ("[ERROR]", error.str())); \
   } while(0)
 
 //static