Propery.hh: fully document unit.
Property: throw MissingResource on lookup error in name-based ctors.
PropertyMessage: rename ADD event to INSTANTIATE.
Throw exception on error instead of returning false/null in: Object::finish_create, ObjectProperty::{instantiate,copy_from,remove,subscribe,_get,_get_field}, Link::subscribe, HUDElementBase::initialize.
Replace remaining uses of MULTI_* macros and remove them: AIActionResultMessage::get_result_data, ObjectProperty::{get,set,get_field,set_field}.
MissingResource: add object-specific constructor.
Fix various typos.