Since 37b0896 stb_image isn't used anymore and recently the nix CI also
broke because of the hacky way the include directory is determined. This
is now fixed. One side effect of this is, that gBar no longer needs to
be cloned recursively.
Add a home-manager module to simplify installation and config management
on nix systems. This greatly reduces the setup time and
makes it integrate better with other programs on nix, mainly Hyprland
which this module is based on.