Simplify GCancellable support on win32

Simplify GCancellable support on win32

There is no need to have a GIOChannel in the GPollFD in
g_cancellable_create_pollfd. All we need is an Event object that
we signal when cancelling and reset when resetting.

Also, supporting g_cancellable_get_fd on Windows using _pipe is useless
as it doesn't work with any corresponding poll() function, so just don't
support that on win32.

I tested this with the cancellation support in GSocket from gnio.