Have ReaderBack be the sole handler of onBack.
Add 4 mutually exclusive options for the Back key,
to avoid ReaderLink and ReaderBack location stacks
from interfering (ReaderBack's stack being always a
superset of ReaderLink's stack).
So, remove "Enable back history", which is replaced
by Back option "Go to previous read page".
Fix a few possible crashes and inconsistencies (when
zoom, scroll or reflow have changed) with ReaderPaging
and ReaderView when restoring previous page/view.
Follow up to b90f6db8: allow specifying an other
value for tap interval when the keyboard is shown
(a good value for tap interval on reader and UI
elements might be too long on the keyboard, and
prevent typing fast).
