cleanup: Use arrow-notation for anonymous functions

Arrow notation is great, but as we only started using it recently,
we currently have a wild mix of Lang.bind(), function() and () => {}.
To make the style consistent again, change all anonymous functions
to arrow notation.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/30

extensions/example/extension.js

@@ -20,7 +20,7 @@ function _showHello() {
     let monitor = Main.layoutManager.primaryMonitor;
     global.stage.add_actor(label);
     label.set_position(Math.floor (monitor.width / 2 - label.width / 2), Math.floor(monitor.height / 2 - label.height / 2));
-    Mainloop.timeout_add(3000, function() { label.destroy(); });
+    Mainloop.timeout_add(3000, () => { label.destroy(); });
 }
 
 // Put your extension initialization code here