Re-export CancelledHandlerId

This type is effectively public, because it’s returned by
`Cancellable::connect_cancelled`, but because it’s not re-exported it
can’t be named, making it awkward to e.g. save it in a struct field.

Fixes GH-1649.

gio/src/lib.rs

@@ -20,7 +20,7 @@ pub use application::{ApplicationBusyGuard, ApplicationHoldGuard};
 mod async_initable;
 mod cancellable;
 mod cancellable_future;
-pub use crate::cancellable_future::{CancellableFuture, Cancelled};
+pub use crate::cancellable_future::{CancellableFuture, Cancelled, CancelledHandlerId};
 mod converter;
 mod credentials;
 mod data_input_stream;