cleanup: Port non-GObject classes to JS6 classes

ES6 finally adds standard class syntax to the language, so we can
replace our custom Lang.Class framework with the new syntax. Any
classes that inherit from GObject will need special treatment,
so limit the port to regular javascript classes for now.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/361

js/ui/checkBox.js

@@ -2,12 +2,8 @@ const Clutter = imports.gi.Clutter;
 const Pango = imports.gi.Pango;
 const St = imports.gi.St;
 
-const Lang = imports.lang;
-
-var CheckBox = new Lang.Class({
-    Name: 'CheckBox',
-
-    _init(label) {
+var CheckBox = class CheckBox {
+    constructor(label) {
         let container = new St.BoxLayout();
         this.actor = new St.Button({ style_class: 'check-box',
                                      child: container,
@@ -28,13 +24,13 @@ var CheckBox = new Lang.Class({
 
         if (label)
             this.setLabel(label);
-    },
+    }
 
     setLabel(label) {
         this._label.set_text(label);
-    },
+    }
 
     getLabelActor() {
         return this._label;
     }
-});
+};