appDisplay: Block search when showing app folder dialogs

Also following design guidance, make the search entry insensitive
when showing an app folder dialog.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/896

js/ui/overview.js

@@ -689,5 +689,9 @@ var Overview = class {
 
         return this.dash.showAppsButton;
     }
+
+    get searchEntry() {
+        return this._overview.searchEntry;
+    }
 };
 Signals.addSignalMethods(Overview.prototype);