Mark extensible enums in winit-core as #[non_exhaustive]

Event, input source, error, data transfer, native key, IME request,
scroll delta and fullscreen enums can now gain variants without a
breaking change. Backends and examples get wildcard arms where they
matched exhaustively; unknown IME requests are reported as
ImeRequestError::NotSupported and unknown fullscreen modes behave
like Fullscreen::Borderless(None).

winit-android/src/event_loop.rs

@@ -977,6 +977,7 @@ impl CoreWindow for Window {
                 *current_caps = None;
                 self.app.hide_soft_input(true);
             },
+            _ => return Err(ImeRequestError::NotSupported),
         }
 
         Ok(())