BUGFIX: Define $tree_class property

Define the `$tree_class` property. This tells the framework that `Dashboard` manages `DashboardPanel` objects. This is used by the Subsites module to check permissions. So is required for compatibility.

code/Dashboard.php

@@ -28,6 +28,10 @@ class Dashboard extends LeftAndMain implements PermissionProvider {
 
 
 	private static $menu_icon = "dashboard/images/dashboard.png";
+
+
+
+	private static $tree_class = 'DashboardPanel';