Collect all MProperties introduced in self

Property definitions

nitc :: model_collect $ Model :: collect_intro_mproperties
	# Collect all MProperties introduced in `self`
	fun collect_intro_mproperties(filter: nullable ModelFilter): HashSet[MProperty] do
		var res = new HashSet[MProperty]
		for mpackage in collect_mpackages(filter) do
			res.add_all mpackage.collect_intro_mproperties(filter)
		end
		return res
	end
src/model/model_collect.nit:204,2--211,4