Support COALESCE and CASE WHEN for subview mappings

[beikov]

Currently we only allow simple or treat mappings, see #327

It would be nice though to also support COALESCE and CASE WHEN. All mappings in a subview are then prefixed with that expression. COALESCE is transformed to CASE WHEN in such a case. For e.g. myAttribute we render CASE WHEN parent IS NOT NULL THEN parent.myAttribute ELSE other.myAttribute END. This only works for basic mappings though i.e. it's not possible to fetch entities this way and this should be validated.

I also don't think that that a @Limit mapping can work for something like that. This shouldn't be an issue though, because the individual parts can define a limit by declaring them as @EntityViewRoot.