[Windows] `dialog.directory` is ignored, always uses current directory
`dialog.directory =` doesn't currently work, it's always the directory containing the app. I think it comes down to this:

https://github.com/trustable-code/NiGui/blob/17c5e0b4024c5741d22257e706f2af2efb7595ad/src/nigui/private/windows/platform_impl.nim#L412

This issue affects both open & save dialogs on Windows - gtk looks fine from the code.

_PS really awesome library with a simple API, thanks for what you've got here so far!_