Add SetDefaultFolder behaviour on Windows

[kiedtl]

I noticed that rfd's set_directory call will always open the dialog in that directory, rather than defaulting to that directory the first time and then opening the last opened directory.

Apparently this is deliberate, to ensure conformity across platforms(?)

Still, having SetDefaultFolder behavior available on Windows would be very useful. So I would like to propose adding a new set_default_directory method that adds this feature.

It'd be Windows-only, however; as far as I can tell this isn't supported by GTK, and I can't test on macOS.