fix #380: regression: the arg parser in the nwjs "open" event callback got broken

This was a consequence of implementing the "-unique" flag. It
required adding a "data-dir" flag to the command line which
through off the _very_ brittle arg parser in the "open" event
callback.

pd/nw/index.js

@@ -313,7 +313,7 @@ function add_events() {
                 argv_string = argv_string.slice(0, -pd_engine_id.length).trim();
                 // strip off the gui dir
                 argv_string = argv_string.slice(0,
-                    -nw.App.argv[3].length).trim();
+                    -nw.App.argv[4].length).trim();
                 if (process.platform === "win32") {
                     // windows quotes this string, so let's remove the two
                     // quotation marks