Eliminated most instances of the legacy setCallbackUserData (and modernized up a bunch of callbacks)

Also cleaned up slfloatermediafilter to act as a proper singleton.
Removes childSetUserData entirely.

indra/newview/llfloaternotificationsconsole.cpp

@@ -263,8 +263,7 @@ BOOL LLFloaterNotification::postBuild()
 		return TRUE;
 	}
 
-	responses_combo->setCommitCallback(onCommitResponse);
-	responses_combo->setCallbackUserData(this);
+	responses_combo->setCommitCallback(boost::bind(&LLFloaterNotification::respond, this));
 
 	LLSD form_sd = form->asLLSD();