Make Notification have toast=true, and UIManager deal
specifically with such widget:
a "toast" widget gets closed by any event, and let
the event be handled by a lower widget.
This should allow us to not wait or tap to get rid
of a notification, and just go on with what we're
doing.
Also make them have a default timeout of 2s,
used with all existing ones.
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.
This implements a reasonable facsimile of going back on Android.
The back button first goes back in a history of visited pages.
When there's no history left, it closes the app.
Fixes#3816.
