These will be used for various purposes described in the previous commit.
EPUBContentSelector selectors are captured for all VS books.  PageSelector
selectors are currently only captured for PDF-based books where we know the page
numbers are "fixed" and should not change depending on viewer settings. We
_might_ be able to capture them for EPUBs as well in future, though we need to
understand more about how reliable/future-proof they are in that context.