Make happytree crossplatform

[whackashoe]

I have started on a branch (very messy) here that aims to port this to linux (mac should be easy from there).

Right now I'm getting black screen with white triangle, so that's a start.

[whackashoe]

So, for file dialogs thinking either some small wrapper (like tinyfiledialog), but then we still need alerts and such.. so perhaps better to use some library like wxwidgets or qt or whatever.

[jarikomppa]

I'd prefer smaller solutions than to move the whole thing to a monstrous framework like qt or wxwidgets =)

[huperniketes]

I've been porting proctree to macOS, but the executable only displays a blue window or screen. I had to modify the shaders to GLSL version 1.2 as that's the version supported on the Mac by default. I'm trying to get it to open an OpenGL 4.1 profile.