Add visibleStart/End properties to HListView that take into account the
ListView's originX/Y, and not just contentX/Y which can be largely
offset.
Use those new properties in EventList for message focusing (fix
ctrl+up/down sometimes incorrectly focusing the message at the middle of
the screen even when we're scrolled at the bottom), and detection of
when we can update the read marker (fix cases when we're scrolled at the
bottom but the read marker never updates).
8d4e67d6a4