small fix for the schema editor so that property deletion is a little bit more robust

before you could add properties with certain names and not be able to remove them at all
Modified contains() builtin function to be able to handle collections of entities.
Unlock read-only properties before setting them. Fixes exception when trying to create new custom-type files.
Fixed failing test. The problem here was that the second argument (result of find(...)) was null and contains(..) return an empty string instead of false.
Added built-in function 'has_relationship'
Merge branch 'master' into structrdb-master