Introduce the concept of "persistent" frames in the sidebar. This is a flag
which can be set on a `Frame` in the sidebar's store to prevent annotations from
being unloaded when the associated guest unloads. This flag is set in response
to a hint provided by the `Guest` in its `documentInfoChanged` message. The
guest in turn gets this hint via `Integration.persistFrame`, which returns true
for VitalSource content frames.

Marking VitalSource content frames as persistent prevents the book's annotations
from being unloaded after a chapter navigation, when the old guest goes away,
and then the same set of annotations immediately being re-fetched from the
server when the new guest connects. This reload of annotations looked ugly but
also lost the user's scroll position in the sidebar. With this change the
sidebar retains the annotations during a chapter navigation and just re-sends
the appropriate ones to the new guest when it connects.