GTK 3 fake dialog action area: fix margins and spacing.

The About box is where it showed up most obviously that I'd hastily
bunged a GtkBox inside another GtkBox without considering their
margins: the 'action area' had twice the margin it should have had and
the rightmost button didn't align with the right edge of the rest of
the window contents.

Easily fixed by giving the inner hbox margin 0 (fixing the right align
and the excessive space around all the buttons), and using the
'spacing' property of GtkBox to ensure multiple buttons in it are
nicely separated without having to take care over that in the client
code that adds them.