overview: Add OverviewActor and use as main actor of the Overlay

Use the Overview class as controller, while create the actual overlay actor
using a GObject-derived class.

Replace actual properties with getter-only properties.

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

js/perf/hwtest.js

@@ -127,7 +127,7 @@ function *run() {
 
     Scripting.scriptEvent('applicationsShowStart');
     // eslint-disable-next-line require-atomic-updates
-    Main.overview._dash.showAppsButton.checked = true;
+    Main.overview.dash.showAppsButton.checked = true;
 
     yield Scripting.waitLeisure();
     Scripting.scriptEvent('applicationsShowDone');