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-appkit/src/dnd.rs

@@ -453,6 +453,7 @@ impl PasteboardWriterState {
             SendData::Uris(_) => None,
             SendData::String(string) => Some(NSString::from_str(&string).into()),
             SendData::Bytes(binary) => Some(NSData::from_vec(binary).into()),
+            _ => None,
         }
     }
 }