A collection of child types of found GXml.CollectionParent objects, used to instantiate the correct class based on the node's name.
The map use the GLib.Type of the object implementing GXml.CollectionParent as the key, to get a GLib.HashTable holding a set of instantiable child classes, with the key as the lowercase of the node's local name to get the GLib.Type of the instantiable class to create and add to the collection.