[ubuntu-art] (Hopefully final) logout dialog

[Manu Cornet]

Hi all !

Sorry to be so verbose about this dialog, but its evolution has been
discussed a lot and I thought it was a good thing to keep you guys
posted. 

I have finally solved all the small problems with the logout dialog (if
you're interested in the precise technical GTK+ solutions, just drop me
a private email). Among other things :

* The amount of space needed by the longest of all "help" strings
(describing what each action actually does) is automatically determined
(no matter which language is used), therefore the dialog no longer
expands while hovering over certain buttons (eg Hibernate).

* Cancel button no longer expands vertically (!).

* Esc closes the dialog.

* Help strings' wording has been enhanced a bit.

I think this should be the final version, except maybe if people in the
Desktop Team have better suggestions for the labels (see previous email
to ubuntu-desktop today).

I've just sent the patch to Sébastien by email, if he is satisfied with
it it should hit the repository in the next dew days :)

Cheers,
Manu