Give plugins a pluginType field

Ordinary plugins have the type "plugin", we'll also have "theme" and
"language"

boot/boot.js

@@ -1038,10 +1038,9 @@ $tw.loadPluginFolder = function(filepath,excludeRegExp) {
 			title: pluginInfo.title,
 			type: "application/json",
 			plugin: "yes",
-			text: JSON.stringify(pluginInfo,null,4)
-		}
-		if("pluginPriority" in pluginInfo) {
-			fields.pluginPriority = pluginInfo.pluginPriority;
+			text: JSON.stringify(pluginInfo,null,4),
+			pluginPriority: pluginInfo.pluginPriority,
+			pluginType: pluginInfo.pluginType || "plugin"
 		}
 		return fields;
 	} else {

themes/tiddlywiki/snowwhite/plugin.info

@@ -3,5 +3,6 @@
 	"description": "A simple, plain layout",
 	"author": "JeremyRuston",
 	"version": "0.0.0",
-	"coreVersion": ">=5.0.0"
+	"coreVersion": ">=5.0.0",
+	"pluginType": "theme"
 }