99e5024e97
This allows us to asynchronously load items into the menu, see
cf98518055. All menus spawned from Python using the `wm.call_menu`
operator will be affected by this. We could avoid that and only refresh
the menu we need to, but it's worth trying to get this to work as a
general menu feature.
This is a slightly risky change, so keeping an eye open for bugs.