`getItem` for lists. Returns mutable for index
Possibity to force set treatment in dynamic trigg

Problem is that change from singular to multiple might not emit `change`
event on relation (it should). Currently there's no easy way to force
that, instead we provide a way to treat value as set, even if at current
point it's not set (more accurately: it's not yet set).
Make sure 'change' event is a trig in not set case