nativefiledialog as submodule

[therustmonk]

PR mlabbe/nativefiledialog#19 was merged. It makes possible to use it as submodule.

@saurvs @emoon Let's consider to use it mentioned way.

Now nativefiledialog (devel branch) uses premake 5 and have Makefiles to popular OS. I offer:

• Use nativefiledialog as git submodule
• Remove nfd folder with local version (and stop to maintain it)
• Update and still use build.rs script

Any ideas? Caveats?

[emoon]

That sounds fine. I very much prefer that we use build.rs as it allows Rust users to use the library without needing external build tools.

[therustmonk]

I've made a PR #3 to fix it.

[therustmonk]

It was merged.