Collect all attributes inherited by self

Property definitions

nitc :: model_collect $ MClass :: collect_inherited_mattributes
	# Collect all attributes inherited by `self`
	fun collect_inherited_mattributes(mainmodule: MModule, filter: nullable ModelFilter): Set[MAttribute] do
		var res = new HashSet[MAttribute]
		for mproperty in collect_inherited_mproperties(mainmodule, filter) do
			if mproperty isa MAttribute then res.add(mproperty)
		end
		return res
	end
src/model/model_collect.nit:855,2--862,4