Recursively walk dbusmenu children-display="submenu" entries when
flattening the SNI menu so the GTK popup can render nested items.
The C side renders submenu folders as labeled buttons that open a
child popup window aligned to the anchor row, kept on-screen with
horizontal flipping; the top-level popup no longer self-destructs
when focus transfers to one of its own submenus.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
68c38247f1