Add Debugging helpers for QMultiHash<K,V>

When debugging with debug helpers enabled, the QHash<K, V> objects can be opened to see the objects they contain.

Unfortunately, this does not work for QMultiHash<K, V> objects: These one just show the QMultiHash<K, V> internals instead of the items inside the QMultihash (see attached screenshots, where m_properties is a QHash<K, V> and m_propertyBindings a QHash<K, V>).

It would be nice to also have the debugging helper also for QMultiHash!