language: Add an image to this page

This is a bit of an experiment, to align the layout of the first
page with the subsequent pages. And what would be a more natural
image for this spot than the GNOME logo?
network: Fix up a spacing mishap
keyboard: Turn into a list box

This is a major refactoring of the input sources page. We
no longer pop up a dialog, instead, we show the input sources
in a list, similar to the way we treat languages and regions
in the previous pages. Only single selection is allowed, and
we don't pay much attention to previous configuration, since
this tool is intended for first-time setup.

Still todo: How to find the 'most common' input sources for
a given locale, and show those initially. This also includes
filtering out 'alternative' approaches such as dvorak.