[Linux] Source configuration/data from standard locations

Presently, GAM uses its own directory structure; however if one wants to package this application for system installation, GAM will not (and should not) have access to alter its installation directories. GAM should be looking in standard Linux configuration directories: `$XDG_CONFIG_HOME/gam/` if available, then `/etc/gam/` for the system defaults. Individual user tokens should be sourced from `$XDG_CONFIG_HOME/gam/` (and the program should refuse to work if the token is world-readable).

These changes would allow packaging into Archlinux for users to install.