Add XDG_DATA_HOME as a directory for resources

[jstsmthrgk]

The builtin resources are installed into /usr/share/emptyepsilon on linux by default.

The user equivalent of that directory is ~/.local/share/emptyepsilon (or $XDG_DATA_HOME/emptyepsilon if $XDG_DATA_HOME is set). So it would make sense to also pick up resources in that directory.

the structure is on purpose

if $XDG_DATA_HOME
    $XDG_DATA_HOME/emptyepsilon
else if $HOME
    $HOME/.local/share/emptyepsilon
if $HOME
    $HOME/.emptyepsilon

so that ~/.emptyepsilon is still picked up no matter if $XDG_DATA_HOME is set or not