Use `prefs.get` directly in events-frame.coffee

Use `prefs.get` directly in events-frame.coffee

... instead of using `messageManager.get`. This avoids synchronous messages,
which is a good thing. It violates the nice principle of only using the parsed
options of the `vimfx` object, but it doesn't really matter right now since only
four prefs are used in events-frame.coffee and they need little to none parsing.
One could probably pass the entire `vimfx.options` object down to frame scripts
when they start up and send updates whenever it updates, but let's do that when
a need for it comes up.