Use the preferred text editor when opening an extension's .js file

Change "./src/gnome-shell --create-extension" to use "gnome-open"
when opening the newly created .js file, so that it is launched
with the user's preferred text editor, instead of hardcoding gedit.

src/gnome-shell.in

@@ -303,7 +303,7 @@ function main() {
 ''')
     f.close()
 
-    subprocess.Popen(['gedit', extensionjs_path])
+    subprocess.Popen(['gnome-open', extensionjs_path])
     sys.exit(0)
 
 if options.eval_file: