Implement preliminary support for XDG Base Directory Specification.

Implement preliminary support for XDG Base Directory Specification.
See http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html.

This feature was requested for a long time (feature requests #1950978 and #2289481).
For now, it is disabled since it breaks compatibility with previous versions.
To be able to test it, you have to enable it by defining USE_XDG to 1 in main.h.

geeqierc and other conf files are going to:
$XDG_CONFIG_HOME/geeqie/ (default to $HOME/.config/geeqie/).

metadata and thumbnails cache (if std is not used) are going to:
$XDG_CACHE_HOME/geeqie/metadata/ (default to $HOME/.cache/geeqie/metadata/)
and $XDG_CACHE_HOME/geeqie/thumbnails/ (default to $HOME/.cache/geeqie/thumbnails/)

collections are going to:
$XDG_DATA_HOME/geeqie/collections/ (default to $HOME/.local/share/geeqie/collections/)

Please test and report any issue.