feat(accessibility): support SetFocus and Press actions on painted list items

[rezabakhshilaktasaraei]

Painted/virtual list items exposed via Accessible::Item advertise themselves as focusable, but they provided no QAccessibleActionInterface — so a screen reader could neither move focus to such an item (UIA SetFocus) nor activate it (UIA Invoke).

This makes Accessible::Item implement QAccessibleActionInterface, but only for children whose owner opts in:

• RpWidget::accessibilityChildSupportsActions(index) (default false) gates the interface; interface_cast returns nullptr otherwise, so unrelated painted lists don't advertise no-op Invoke / SetFocus / Select on Windows.
• actionNames() advertises pressAction(), setFocusAction() (when the row is focusable) and toggleAction() (when selectable — Qt routes UIA SelectionItem.Select through it on the Qt 5 bridge).
• doAction() routes press → accessibilityChildActivate, focus/toggle → accessibilityChildSetFocus.

Actions are dispatched by a stable identity, not by index: the owner provides accessibilityChildIdentity(index) / accessibilityChildIndexByIdentity(token), and the action virtuals take that token. The owner resolves it to the current row on the main thread, so a reorder or replacement can't make a queued action hit the wrong row. A cached item is also dropped when the row at its index changes identity.

All the new RpWidget virtuals default to no-ops / empty, so a widget opts in explicitly (e.g. the chat list).