MenuItem API exposes checkable property directly

[saintmatthieu]

Resolves: audacity/audacity#8268

The AbstractMenuModel implements its entire tree with MenuItems, which only allow setting the checkmark via an Action.
While it makes some sense that menu leaves are associated with actions, it does not for their parents, and hence MenuItems should give access to their checked state without having to modify the action (at least not at API level).

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