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/changelog/unreleased.md

@@ -163,6 +163,7 @@ changelog entry.
 - On X11, use bottom-right corner for IME hotspot in `Window::set_ime_cursor_area`.
 - On macOS and iOS, no longer emit `ScaleFactorChanged` upon window creation.
 - On macOS, no longer emit `Focused` upon window creation.
+- On iOS, emit more events immediately, instead of queuing them.
 
 ### Removed