Insert a new supported instantiatable type in given hash table, by instantiating the type, getting its node's local name as key.
| types |
a GLib.HashTable to hold supported types |
| parent_type |
a GLib.Type as parent of supported types, it is not necesarry to be an instantiatable type, like interfaces, should be the same of GXml.Collection.items_type |
| type |
a supported instantiatable GLib.Type to be added in the collection |