fix(17): WR-01 keep a menu entry's subtree and parent in its menu

- The admin refuses a parent from another menu than the one the form
  names (errors.parent_menu) instead of moving the entry to the parent's
  menu without a word; BeforeSave records the named menu
  (MenuEntry.RequestedMenuID).
- An entry moved to another menu moves its descendants in the same
  transaction, so the public tree no longer drops them.

lang/en/lang.yaml

@@ -19,7 +19,7 @@ models:
     label: Menu entry
     label_plural: Menu entries
     menu: Menu
-    menu_comment: Entries with a parent belong to the parent's menu.
+    menu_comment: Entries with a parent belong to the parent's menu; leave it empty to take the parent's. Moving an entry moves its sub-entries with it.
     parent: Parent
     type: Type
     type_url: Link
@@ -45,3 +45,4 @@ errors:
   type_unknown: Choose one of the listed types.
   menu_required: Choose a menu or a parent entry.
   parent_cycle: An entry cannot be placed under itself or one of its own entries.
+  parent_menu: The parent entry belongs to another menu. Choose a parent from the selected menu, or change the menu.