Move node-webkit saver into a plugin

We don’t need it for TiddlyDesktop now that it reuses the TiddlyFox
saver. But it is still useful for embedding a TW directly into a
node-webkit app

plugins/tiddlywiki/nodewebkitsaver/nodewebkit.js

@@ -3,7 +3,7 @@ title: $:/core/modules/savers/nodewebkit.js
 type: application/javascript
 module-type: saver
 
-Handles saving changes in the node-webkit environment
+Handles saving changes in the node-webkit environment. Not required by TiddlyDesktop, which re-uses the TiddlyFox saver, but useful if you're embedding a single TiddlyWiki document into a node-webkit app.
 
 \*/
 (function(){

plugins/tiddlywiki/nodewebkitsaver/plugin.info

@@ -0,0 +1,6 @@
+{
+	"title": "$:/plugins/tiddlywiki/nodewebkitsaver",
+	"description": "Native saving for node-webkit",
+	"author": "JeremyRuston",
+	"core-version": ">=5.0.0"
+}