workspacesView: Enable keynav between all monitors

Instead of using one focus group for each (per-monitor) workspacesView,
use a single one that is shared between all of them.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/412

js/ui/workspacesView.js

@@ -33,7 +33,6 @@ var WorkspacesViewBase = new Lang.Class({
         this.actor = new St.Widget({ style_class: 'workspaces-view',
                                      reactive: true });
         this.actor.connect('destroy', this._onDestroy.bind(this));
-        global.focus_manager.add_group(this.actor);
 
         // The actor itself isn't a drop target, so we don't want to pick on its area
         this.actor.set_size(0, 0);
@@ -428,6 +427,10 @@ var WorkspacesDisplay = new Lang.Class({
         this.actor.connect('notify::allocation', this._updateWorkspacesActualGeometry.bind(this));
         this.actor.connect('parent-set', this._parentSet.bind(this));
 
+        this._viewsContainer = new St.Widget();
+        Main.layoutManager.overviewGroup.add_actor(this._viewsContainer);
+        global.focus_manager.add_group(this._viewsContainer);
+
         let clickAction = new Clutter.ClickAction();
         clickAction.connect('clicked', action => {
             // Only switch to the workspace when there's no application
@@ -579,7 +582,7 @@ var WorkspacesDisplay = new Lang.Class({
             }
 
             this._workspacesViews.push(view);
-            Main.layoutManager.overviewGroup.add_actor(view.actor);
+            this._viewsContainer.add_actor(view.actor);
         }
 
         this._updateWorkspacesFullGeometry();