Make cancellable arguments consistent with gio-rs Option<&Cancellable>
Currently we have
pub fn read_stream<'a, P: Into<Option<&'a Cancellable>>>( self, stream: &gio::InputStream, base_file: Option<&gio::File>, cancellable: P, ) -> Result<SvgHandle, LoadingError>;
@sdroege I thought I had taken notes on our conversation about the correct/future way to pass cancellables to gio, but of course I forgot to copy from IRC into my notes buffer :( Can you please remind me what's the thing to do here?
(We already have #446 (closed) for the