Dynamic optional view

Being able to use `Option<View>` as a view is very convenient but does not allow for the state Some/None to change. Being able to dynamically change this would be really useful. I've been able to emulate something like this by stitching `Cond` together with a helper `Unwrap` ViewTemplate. So looks pretty doable to me but I've yet to fully wrap my head around the state tracking in quill works.