Provide access to window/widget winId via GafferUI api

[bentoogood]

To help with #143

[johnhaddon]

I don't think we should add any public methods for retrieving this - the GafferUI API is designed to shield the user from the implementation details, including Qt and system level stuff like this. This allows us to change the implementation in the future (GafferUI started life using GTK).

If you really want to do things via a winId(), and you're happy for your code to be Qt specific, you should be able to do what you want already using myGafferWidget._qtWidget().winId().

Alternatively I'm happy to add an API in GafferUI which takes a GafferUI.Widget and returns an IECore.ImagePrimitive screen capture of it - that wouldn't expose any implementation details.