Switch to using tags to identify stylesheets

Using module-type was confusing, because we weren't actually using the
module infrastructure for stylesheet processing.

core/modules/startup.js

@@ -90,7 +90,8 @@ exports.startup = function() {
 			$tw.crypto.setPassword(null);
 		});
 		// Apply stylesheets
-		$tw.utils.each($tw.modules.types.stylesheet,function(moduleInfo,title) {
+		var stylesheetTiddlers = $tw.wiki.filterTiddlers("[tag[$:/tags/stylesheet]]");
+		$tw.utils.each(stylesheetTiddlers,function(title,index) {
 			// Stylesheets don't refresh, yet
 			var parser = $tw.wiki.parseTiddler(title),
 				renderTree = new $tw.WikiRenderTree(parser,{wiki: $tw.wiki});

themes/tiddlywiki/snowwhite/base.tid

@@ -1,5 +1,5 @@
 title: $:/core/styles/base
-module-type: stylesheet
+tags: [[$:/tags/stylesheet]]
 
 \rules only filteredtranscludeinline transcludeinline macrodef macrocallinline
 \define border-radius(radius)