ReaderLink: make all links be a table (they were a table for PDF,
but a string for CRE) for clearer code. Also have location_stack
store them as tables, with additional properties.
Get original position of link source (and verify it is valid)
so we can show a marker there.
Also:
Hold on "Go back to previous location" to clear location stack.
Resists "Swipe to go back" when previous locations stack has just
become empty, and show a notification.
Fix wrong links with Swipe to follow nearest link on PDF documents.