qmlRegisterType types never go away

When types are registered using qmlRegisterType there is no way to unregister them save for restarting the application. This leads to problems when you want to change the types available to the application
1) Re-registered types on top of the same uri/version/name lead to a memory leak because the inaccessible previous type information is still there
2) There is no way to remove a Type save for overwriting it with an invalid type registration (which still leaves a memory leak).

Some API should be added to allow flushing of the global type cache.