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

tests/interactive/calendar.js

@@ -17,7 +17,7 @@ function test() {
     // Calendar can only be imported after Environment.init()
     const Calendar = imports.ui.calendar;
     let calendar = new Calendar.Calendar();
-    vbox.add(calendar.actor,
+    vbox.add(calendar,
              { expand: true,
                x_fill: false, x_align: St.Align.MIDDLE,
                y_fill: false, y_align: St.Align.START });