Detect and display info about old OpenGL versions.

[nsg]

Find a good cross platform way to display a non-OpenGL GUI-dialogue that informs the user that the game require at least OpenGL 3.3.

Ref #223 and #219

[nsg]

https://github.com/aaronmjacobs/Boxer is an interesting choice to try out. But it only supports Windows, GTK 3 and OSX.

Do we like to have a hard dependency on GTK 3? I guess it's fine consider that most people will probably have that. Finally, it would be nice to have a fallback solution (maybe just use stdout) for other platforms like FreeBSD, define or compile flag will probably do just fine.

[TamaMcGlinn]

How 'bout putting this in the readme of the repository? It's a bit frustrating to just have a segfault immediately upon startup. All I can execute is ./konstructs -h - but that help message also doesn't specify that I needed OpenGL 3.3