Multiple screen configurations support

[Marc MAURICE]

Hi all,

I'm currently working on a patch in xfce4-settings to support multiple 
screen configurations.
(like it's done in gnome settings / ubuntu)

Because currently configuration is just saved in a "Default" scheme, and 
is specific for each ouput.
Which mean you can't have 2 different configs if pluging 2 different 
screens on the same output.