Implement XDG_CONFIG_HOME resolution in shell to get rid qtpaths dependency.

Also used in other places of xdg-mime because the qtpaths replacement implements the base-dir specification more correctly than previous shell hackery.

The get_xdg_config_home is implemented in the common file as xdg-settings and xdg-desktop-menu could benefit from this implementation too.

Fixes: #258

@gudvinr and @arojas: Does that fix work for you?