unhideOnNextDisplay used by two threads without synchronization

Two threads are accessing `unhideOnNextDisplay` [in FilteredView.swift](https://github.com/michelf/sim-daltonism/blob/99a61293a063b7588404b4ab5e286f3ee38b0bef/Mac%20App/FilteredView.swift#L225-L228) without proper synchronization. Even though it seems work, it is theoretically undefined behavior and should be fixed.