iOS: Never queue application-level events (#3905)

Events like `resumed`, `new_events`, `about_to_wait`, and so on will
never happen as a result of programmer action, so we'll never need to
queue those. This allows us to remove the need for the old `Event`
struct in the iOS backend.

Furthermore, we can now remove `InUserCallback`, since that state is
already stored inside `EventHandler`.

I've tried to otherwise keep the semantics as close to the original by
calling `handle_nonuser_events(mtm, [])`, which flushes pending events.

src/platform_impl/apple/event_handler.rs

@@ -106,7 +106,6 @@ impl EventHandler {
         self.inner.try_borrow().is_err()
     }
 
-    #[cfg(target_os = "macos")]
     pub(crate) fn ready(&self) -> bool {
         matches!(self.inner.try_borrow().as_deref(), Ok(Some(_)))
     }