add default keybinding for screenshooter in keyboard plugin

[Brian J. Tarricone]

Jérôme Guelfucci wrote:

> 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.

No, definitely not.  The goal here should be to make xfce-utils as small 
as possible, not bloat it with stuff people may not need.  I'd be in 
favor of patches that push us toward completely eliminating xfce-utils, 
not things that make it larger.

	-brian