[PATCH 6/6] WIP: support XDG database directory

[Tomi Ollila]

This is good opening for (eventually "fixing" e.g. library interface...);
in its current state I can come up 2 (easily solvable) problems

1) the xapian database holding email indexes can grow to be quite large;
user may have large space for emails (somewhere else than HOME) but small
HOME. This can be "fixed" by keeping the database still in the same hierarchy
as email files. Making user to point XDG_DATA_HOME elsewhere is (I) global
and (II) fragile for an user to do.

2) User may have multiple separate email configurations under one user
account; This is easy to fix with environment variable; if exists,
overrides XDG_DATA_HOME or $HOME/.local/share -- actually if this holds
"only" configurations, the XDG_CONFIG_HOME is more appropriate
(again, in simple case users should not be bothered to set any environment
variables themselves, but should be able to survive with large databases).

(standard disclaimer apply :)

Tomi


PS: I have not (yet) looked the other patches; been too
busy writing code to my personal htpc environment...