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/panelMenu.js

@@ -2,7 +2,6 @@
 /* exported Button, SystemIndicator */
 
 const { Atk, Clutter, GObject, St } = imports.gi;
-const Signals = imports.signals;
 
 const Main = imports.ui.main;
 const Params = imports.misc.params;
@@ -200,24 +199,34 @@ var Button = GObject.registerClass({
  * of an icon and a menu section, which will be composed into the
  * aggregate menu.
  */
-var SystemIndicator = class {
-    constructor() {
-        this.indicators = new St.BoxLayout({ style_class: 'panel-status-indicators-box',
-                                             reactive: true });
-        this.indicators.hide();
+var SystemIndicator = GObject.registerClass({
+    GTypeName: 'PanelMenu_SystemIndicator',
+}, class SystemIndicator extends St.BoxLayout {
+    _init() {
+        super._init({
+            style_class: 'panel-status-indicators-box',
+            reactive: true,
+            visible: false
+        });
         this.menu = new PopupMenu.PopupMenuSection();
     }
 
+    get indicators() {
+        let klass = this.constructor.name;
+        let { stack } = new Error();
+        log(`Usage of indicator.indicators is deprecated for ${klass}\n${stack}`);
+        return this;
+    }
+
     _syncIndicatorsVisible() {
-        this.indicators.visible = this.indicators.get_children().some(a => a.visible);
+        this.visible = this.get_children().some(a => a.visible);
     }
 
     _addIndicator() {
         let icon = new St.Icon({ style_class: 'system-status-icon' });
-        this.indicators.add_actor(icon);
+        this.add_actor(icon);
         icon.connect('notify::visible', this._syncIndicatorsVisible.bind(this));
         this._syncIndicatorsVisible();
         return icon;
     }
-};
-Signals.addSignalMethods(SystemIndicator.prototype);
+});