cleanup: Use inheritance for Actor classes instead of composition

Remove the `this.actor = ...` and `this.actor._delegate = this` patterns in most
of classes, by inheriting all the actor container classes.

Uses interfaces when needed for making sure that multiple classes will implement
some required methods or to avoid redefining the same code multiple times.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559

js/ui/checkBox.js

@@ -1,16 +1,19 @@
 /* exported CheckBox */
-const { Clutter, Pango, St } = imports.gi;
+const { Clutter, GObject, Pango, St } = imports.gi;
 
-var CheckBox = class CheckBox {
-    constructor(label) {
+var CheckBox = GObject.registerClass(
+class CheckBox extends St.Button {
+    _init(label) {
         let container = new St.BoxLayout();
-        this.actor = new St.Button({ style_class: 'check-box',
-                                     child: container,
-                                     button_mask: St.ButtonMask.ONE,
-                                     toggle_mode: true,
-                                     can_focus: true,
-                                     x_fill: true,
-                                     y_fill: true });
+        super._init({
+            style_class: 'check-box',
+            child: container,
+            button_mask: St.ButtonMask.ONE,
+            toggle_mode: true,
+            can_focus: true,
+            x_fill: true,
+            y_fill: true
+        });
 
         this._box = new St.Bin();
         this._box.set_y_align(Clutter.ActorAlign.START);
@@ -32,4 +35,4 @@ var CheckBox = class CheckBox {
     getLabelActor() {
         return this._label;
     }
-};
+});