Add support for Window.SetIcon(icon) (win + gtk only)

Hi!

I had a use-case where I wanted to be able to dynamically change the app icon at runtime.  This patch supports the functionality.

Tested on win32 only
Should support GTK (it's a very simple patch)
MacOS doesn't seem to support setting the icon at all (even from package.json) so it's left as a no-op there.

Thanks for reviewing this!