Add "Build FGD" button (C++)

[retro-git]

trafficstars

I saw that #55 adds a "Build FGD" button to export an fdg definition file. However, since that code is written in GDScript, I have rewritten it in C++ to be more in-line with this project's style and to clean up the code somewhat.

[codecat]

Hey this is pretty cool! I haven't looked too much at this yet, but maybe @krazyjakee has some input on this?

[krazyjakee]

That's all hieroglyphs to me. Much like my own version, it really needs tests to provide assurance. Not a necessity of course but my 2 cents.