Partly revert "Use `prefs.get` directly in events-frame.coffee"

Partly revert "Use `prefs.get` directly in events-frame.coffee"

This partly reverts commit e7f361942c2b07234e889804975e93c21c1f5baf.

That commit switched to using `prefs.get` directly in
events-frame.coffee instead of using `messageManager.get`. That avoids
synchronous messages, which is a good thing.

However, I forgot that when doing so, `vimfx.addOptionOverrides` won't
take effect.

This commit partly reverts to getting options via synchronous messages
again. In the very frequent 'keydown' event direct `prefs.get` is still
used for the unlikely-to-be-overridden `focus_previous_key` and
`focus_next_key` prefs, while synchronous messaging is used in the less
frequent 'focus' event, which enables the use case of overriding
`prevent_autofocus` on select pages.