Collect all properties introduced in self

Property definitions

nitc :: model_collect $ MPackage :: collect_intro_mproperties
	# Collect all properties introduced in `self`
	fun collect_intro_mproperties(filter: nullable ModelFilter): HashSet[MProperty] do
		var res = new HashSet[MProperty]
		for mgroup in mgroups do
			for mmodule in collect_all_mmodules(filter) do
				res.add_all mmodule.collect_intro_mproperties(filter)
			end
		end
		return res
	end
src/model/model_collect.nit:350,2--359,4