Bump the Vala and GTK+ requirements

We'll definitely need GTK+ 3.12 for all the new header bar properties
(e.g. GtkDialog's use-header-bar).

I don't think we need to require Vala 0.24 yet, so let's stick with 0.22
for now. (I'm not even sure we necessarily need 0.22 -- probably not --
but 0.16 is almost surely too old.)