Support XDG basedir specification

[Eric Heintzmann]

Please support FreeDesktop.org XDG basedir specification for guvcview

http://ploum.net/post/207-modify-your-application-to-use-xdg-folders
https://live.gnome.org/GnomeGoals/XDGConfigFolders
http://standards.freedesktop.org/basedir-spec/latest/

[Eric Heintzmann]

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 guvcview 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.

[Paulo Assis]

The only user specific config produced by guvcview is ~/.guvcviewrc (or ~/.guvcviewrc-videoN... for other video devices besides the default video0).

So according to XDG Base directory specification these should go into something like ~/.config/guvcview/videoN ?

This change should be really easy to implement, but what about old configs? should guvcview still check for them (at least for the next couple of versions)? Or do we just drop support for the old config files altogether?