allow admin components to skip being shown in the admin menu

This is related to #4249. At least the farmer plugin provides a top
level admin component and several sub components which are loaded from
the top component. For this use case it makes sense to be able to hide
components from the admin interface. Default remains to show all.

inc/Extension/AdminPlugin.php

@@ -59,6 +59,16 @@ abstract class AdminPlugin extends Plugin
         return 1000;
     }
 
+    /**
+     * Return true if the plugin should be shown in the admin menu
+     *
+     * @return bool
+     */
+    public function showInMenu()
+    {
+        return true;
+    }
+
     /**
      * Carry out required processing
      */

inc/Ui/Admin.php

@@ -135,6 +135,7 @@ class Admin extends Ui
 
             // check permissions
             if (!$obj->isAccessibleByCurrentUser()) continue;
+            if (!$obj->showInMenu()) continue;
 
             if (in_array($p, $this->forAdmins, true)) {
                 $type = 'admin';