ModelIndex used to perform searches

Property definitions

nitc :: model_index $ Model :: _index
	# ModelIndex used to perform searches
	var index: ModelIndex is lazy, writable do
		var index = new ModelIndex
		for mentity in collect_mentities do
			if mentity isa MClassDef or mentity isa MPropDef then continue
			index.index mentity
		end
		return index
	end
src/model/model_index.nit:144,2--152,4