Move admin_menu to Redmine::MenuManager
Admin menu should be moved to a standard Redmine::MenuManager so that it can be modified in a plugin the same way we do with other menu.
For the moment the only way to modify this menu really (not just add something, reorganize or remove items for instance) is to override the view. If 2 plugins do this, the 2nd will override 1st modifications. This problem doesn't occur anymore if the menu is managed with Redmine::MenuManager.
See patch attached. If it's functionnally OK, we can integrate this easily, it should not break anything in existing plugins.