This is useful to disable items that are not applicable right now because of
some condition (e.g. the "delete branch" menu item when the currently
checked-out branch is selected).
When a DisabledReason is set on a menu item, we
- show it in a tooltip (below the regular tooltip of the item, if it has one)
- strike through the item's key, if it has one
- show an error message with the DisabledReason if the user tries to invoke the
command
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