This makes sense because the pages to export checkboxes are here. The
export modal should be sort of a "last-chance" settings thing... not
actual core settings.
This is done using the new checkboxes on the page list under the World
panel. Seemed like the best way to do it within our format. Perhaps the
rest of the exporter options should move to that panel at some point.
We are attempting to encourage the pages=layers paradigm; however, this is
not strictly enforced. This commit has lots of bits and bobs but little
actual substance. More to come when I begin work on the exporter.