add default keybinding for screenshooter in keyboard plugin

[Jérôme Guelfucci]

Hello,

I would like to add a new default keybinding to
http://svn.xfce.org/svn/xfce/xfce-mcs-plugins/trunk/plugins/keyboard_plugin/default.xml
to have the PrtScrn key working with xfce4-screenshooter-plugin. I've
added an executable to the plugin which makes possible to take
screenshots of the desktop and of the active window with a command
line.

I suggest the following keybindings:

PrtScreen -> launch xfce4-screenshooter
Alt + PrtScreen -> launch xfce4-screenshooter -w

I also propose to add the executable code to xfce-utils, so that one
can take screenshots with a default installation of xfce, without
installing another plugin.

Thank you in advance,

Jérôme