popupMenu: release destroyed item signals

[H234598]

Summary

• disconnect every parent-managed signal owned by destroyed menu items, child menus, and sections
• replace per-item parent state handlers with one menu-lifetime handler
• preserve sibling submenu and separator behavior
• recurse through PopupMenuSection when updating item state

The previous cleanup left the managed destroy signal behind, retaining destroyed items. Submenu destruction could also disconnect unrelated parent open-state-changed handlers.

Closes #13956

Tests

• 1000 add/destroy cycles for PopupMenuItem, PopupSeparatorMenuItem, PopupSubMenuMenuItem, and PopupMenuSection
• sibling submenu behavior after destroying another submenu
• separator update after destroying an adjacent submenu
• submenu close inside nested sections
• non-animated and animated/deferred parent close paths
• isolated Xvfb/D-Bus runner, including forced outer-timeout cleanup
• exact base revision as negative control
• exact 6.6.9 source-port run

Run locally:

./tests/run-popup-menu-signals.sh