Support XDG basedir specification for FontForge

According to XDG Base directory specification:
User data should go into $XDG_DATA_HOME (which default to ~/.local/share),
user preferences should go into $XDG_CONFIG_HOME (which default to ~/.config)
and cached data should go to $XDG_CACHE_HOME (which default to ~/.cache).
More details at :
http://ploum.net/post/207-modify-your-application-to-use-xdg-folders
https://live.gnome.org/GnomeGoals/XDGConfigFolders

Full specification can be found at:
http://standards.freedesktop.org/basedir-spec/latest/

The Freedesktop.org XDG base directory specification have good de facto adoption.
It has been adopted by:
- GNOME ( https://live.gnome.org/GnomeGoals/XDGConfigFolders )
- GTK+ ( https://bugzilla.gnome.org/show_bug.cgi?id=646631 )
- KDE ( http://techbase.kde.org/KDE_System_Administration/XDG_Filesystem_Hierarchy#Freedesktop.org_and_Standard_Directories )
- QT ( http://harmattan-dev.nokia.com/docs/library/html/qt4/qsettings.html#setPath )
- XFCE ( http://docs.xfce.org/xfce/xfce4-session/advanced in Files and Environment Variables )
- LXDE
- Razor-qt
- VLC ( https://trac.videolan.org/vlc/ticket/1267 )
- GStreamer ( https://bugzilla.gnome.org/show_bug.cgi?id=518597 )
- Chrome ( http://code.google.com/p/chromium/issues/detail?id=16976 )
- many more upstream applications
- Ubuntu ( http://brainstorm.ubuntu.com/idea/6557/ & http://packages.ubuntu.com/fr/source/precise/libxdg-basedir )
- Debian ( http://packages.debian.org/squeeze/libxdg-basedir1 )
- Red Hat
- Fedora
- Suse
- many more distributions

I think that FontForge should use same locations than the vast majority of Desktop environment and applications.

There are real advantages of following this specification :
- a lot less cluttered $HOME
- Make backups a lot more safer and easier.
Backuping your $XDG_DATA_HOME along with your files is enough
(or just excluding $XDG_CACHE_HOME)
- A lot easier to reset a default configuration if you want/need it (and
without any risk to loose informations). Even for the software itself
could choose to reset $XDG_CONFIG_HOME if needed.
- Avoid some strange bugs that happens because you had a old version of
some configuration file
- A lot more of flexibility and portability because no path are hardcoded.