Avoid using the sitemap.config file when creating a menu in the admin area

[skoshelev]

We switched to a new menu in the administration area long ago. However, we still use the inconvenient approach of storing the tree structure in the XML file.

It is necessary to rewrite this mechanism to a simpler one, using classes and services. We should also abandon IAdminMenuPlugin in favor of using events. Both of these steps will simplify the logic of adding new menu items to the administration area and add control over changes