cleanup: Use Function.prototype.bind()

When not using arrow notation with anonymous functions, we use Lang.bind()
to bind `this` to named callbacks. However since ES5, this functionality
is already provided by Function.prototype.bind() - in fact, Lang.bind()
itself uses it when no extra arguments are specified. Just use the built-in
function directly where possible, and use arrow notation in the few places
where we pass additional arguments.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/23

js/ui/background.js

@@ -539,7 +539,7 @@ var BackgroundSource = new Lang.Class({
         this._backgrounds = [];
 
         this._monitorsChangedId = global.screen.connect('monitors-changed',
-                                                        Lang.bind(this, this._onMonitorsChanged));
+                                                        this._onMonitorsChanged.bind(this));
     },
 
     _onMonitorsChanged() {