Redefine as defun

Don't inline `folio-spellcheck-remove-local-hooks'; it should be
possible to redefine this function.
Add hook for tracking updates to page labels

Add the abnormal hook `folio-page-label-changed-functions' for
tracking updates to page labels.  This hook normally is run from the
project page for setting up the pagination of a book.
Add optional args to `folio-toggle-nobreak-char-display'

The menu-bar stuff seems to depend on that function having an
argument; if not that I don't know.  Adding an optional argument
silences the compiler at least.
Add functions for installing local mode hooks

Add `folio-core-add-local-hooks' and `folio-core-remove-local-hooks'
for registering and deregistering buffer local mode hooks.
Install and deinstall mode hooks

When setting up the folio mode core register buffer local hooks;
deregister when tearing it down.
Move definition to precede first use

Move the definition of `folio-page-label-rule-unfold' to precede its
first use in this package and properly support blank pages.  The
latter are defined to have a page label assigned with the value of t.