Add `folio-thing-at-point-syntax-table' for parsing Folio mode
'things', semantic elements with an sexp-like syntax.
Add the general `folio-forward-note-thing' defun for moving over
note-like things such as side-note, foot-notes, proofer's note,
etc. Makes use of `folio-thing-at-point-syntax-table'.
Add the semantic element of a blank page and movement functions for
both directions.
Add the semantic element of an illustration and associated movement
functions.
Add the semantic element of a footnote and associated movement
functions.
Add the semantic element of a side-note and associated movement
functions.