Please use standard $XDG_CACHE_HOME for cache

[bertrandmarc]

Dear Pingus developpers,

According to the XDG Base Directory Specification [1], "user-specific non-essential (cached) data should be written" in a directory defined by the environment variable $XDG_CACHE_HOME. "If $XDG_CACHE_HOME is either not set or empty, a default equal to $HOME/.cache should be used."

This would be a great help to avoir backup software (see [2] for more details) to save pingus cache (currently in ~/.pingus/cache).

Best regards,
Bertrand

[1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
[2] http://www.brynosaurus.com/cachedir/