Update asset_cache to modern /tg/, and make TGUI compatible with it. CDN support, etc.

code/modules/asset_cache/asset_cache_item.dm

@@ -0,0 +1,23 @@
+/**
+	* # asset_cache_item
+	* 
+	* An internal datum containing info on items in the asset cache. Mainly used to cache md5 info for speed.
+**/
+/datum/asset_cache_item
+	var/name
+	var/url
+	var/md5
+	var/resource
+
+/datum/asset_cache_item/New(name, file)
+	if (!isfile(file))
+		file = fcopy_rsc(file)
+	md5 = md5(file)
+	if (!md5)
+		md5 = md5(fcopy_rsc(file))
+		if (!md5)
+			CRASH("invalid asset sent to asset cache")
+		log_world("asset cache unexpected success of second fcopy_rsc")
+	src.name = name
+	url = name
+	resource = file