TGUI and asset cache sending improvements (#43090)

Pills are now two spreadsheets instead of individual icons.

This reduces assets sent by about 19-20.

Furthermore tgui has been extended with a new proc ui_base_html which
allows a user to customise the html sent as the base for a tgui window.
This allows someone to provide custom spritesheet css as links in a
specific tgui window without having to add it to the global html.

code/modules/tgui/external.dm

@@ -49,6 +49,20 @@
 	if(!ui || ui.status != UI_INTERACTIVE)
 		return 1 // If UI is not interactive or usr calling Topic is not the UI user, bail.
 
+ /**
+  * public
+  *
+  * Called on an object when a tgui object is being created, allowing you to customise the html
+  * For example: inserting a custom stylesheet that you need in the head
+  *
+  * For this purpose, some tags are available in the html, to be parsed out with replacetext
+  * (customheadhtml) - Additions to the head tag
+  *
+  * required html the html base text
+  *
+ **/
+/datum/proc/ui_base_html(html)
+	return html
 
  /**
   * private

code/modules/tgui/tgui.dm

@@ -198,11 +198,19 @@
  **/
 /datum/tgui/proc/get_html(var/inline)
 	var/html
+	html = SStgui.basehtml
+
+	//Allow the src object to override the html if needed
+	html = src_object.ui_base_html(html)
+	//Strip out any remaining custom tags that are used in ui_base_html
+	html = replacetext(html, "<!--customheadhtml-->", "")
+
 	// Poplate HTML with JSON if we're supposed to inline.
 	if(inline)
-		html = replacetextEx(SStgui.basehtml, "{}", get_json(initial_data))
-	else
-		html = SStgui.basehtml
+		html = replacetextEx(html, "{}", get_json(initial_data))
+
+
+	//Setup for tgui stuff, including styles
 	html = replacetextEx(html, "\[ref]", "[REF(src)]")
 	html = replacetextEx(html, "\[style]", style)
 	return html