Update/Modernize LLAvatarName and LLAvatarNameCache and _EVERYTHING_ they touch

Fairly certain this adds the feature in which the user may click the online notification to open a chat with the user coming online.

Translators may want to look at the xml changes of this change to see what to update

Removes old LegacyMultiAttachmentSupport patch
Moves LLAvatarName-centric functions into LLAvatarName from elsewhere.
Adds boost::signals2::connections to name polling places.
Removes more old icky static callbacks
Merge from Alchemy, credit where due~
Update notifications.xml to have a bunch of the tags from upstream to ease diffing

indra/newview/llimview.cpp

@@ -1091,7 +1091,7 @@ void LLIMMgr::noteOfflineUsers(
 			std::string full_name;
 			if (info
 				&& !info->isOnline()
-				&& LLAvatarNameCache::getPNSName(ids.get(i), full_name))
+				&& LLAvatarNameCache::getNSName(ids.get(i), full_name))
 			{
 				LLUIString offline = LLTrans::getString("offline_message");
 				offline.setArg("[NAME]", full_name);