add default keybinding for screenshooter in keyboard plugin

[Jérôme Guelfucci]

> BTW, I just found out it doesn't work(setting the keybinding), at least on
> my system. Apparently calling the executable from command-line has no
> effect, which is expected if you think that is a panel plugin.... Or am I
> missing something?
>
> --
> Cheers,
> Rodrigo
>
> A computer is like air conditioning: it becomes useless when you open
> windows.
> ~Linus Torvalds
> _______________________________________________

Please test with latest trunk and file a bug if something does not work.

Thank you !

Jérôme