Reduce the spinner sizes to 32px

Latest GTK+ only supports spinners up to 32px; if we request a larger
size it just gets used as padding and messes up the layout.

We could however create a custom spinner (with a custom rotating image,
should be rather easy) if the designers think it's better to have a
larger spinner.
Use different wording for first run dialog's button

Use "Let's Go Shopping" instead of "Let's Get Started", as in the
original mockup in https://bugzilla.gnome.org/show_bug.cgi?id=731973

12:10 < aday> using the word shopping is actually really helpful - it
encapsulates what you do with Software (except for the payment part)
12:11 < aday> "started" isn't instructive
When using --prefer-local resize, pad and sharpen the images just like the AppStream builder does