Support XDG Base Directory Specification

[a-andre]

XDGBDS would help to distinguish between user data, user config and cache files. Currently it's all in DOT_SAGE and the user don't know which of these files can safely be removed without losing worksheets, for example.

See https://wiki.debian.org/XDGBaseDirectorySpecification for details.

We might use https://pypi.python.org/pypi/pyxdg to implement this.

Component: misc

Keywords: xdg, DOT_SAGE

Issue created by migration from https://trac.sagemath.org/ticket/17185