Follow XDG Base directory specs

[MckinleyX]

Hi!
On Linux, elan stores files in ~/.elan.

It may be better to follow the XDG desktop specification.

In particular,
Configuration files should go under $XDG_CONFIG_HOME (if empty, $HOME/.config/)
Data files should go under $XDG_DATA_HOME (if empty, $HOME/.local/share/)

[sbatial]

I agree, that the XDG standard should probably be the default, but it is already possible to set ELAN_HOME.