Now we can parametrize Higan locally.
By default, guiToolkit = "gtk" and profile = "performance" (the accuracy
profile is seriously slow on my machine :) )
Closes#4340
Higan uses a lot of hardcoded paths when it looks for runtime files
as shaders, icons and mainly BIOS files. So, we made a q&d wrapper
script: it copies all these files to $HOME/.config/higan in the 1st
invocation.
Higan is a cycle-accurate Nintendo multi-system emulator
It is a preliminary release for Nix - I need to investigate
some issues about install process and hardcoded paths...
