Share a static invalid QtModelIndex between jvm & native

In many interfaces of QtAbstractModelIndex and its proxies, an invalid QtModelndex goes back and forth between jvm & native. This needs unnecessary JNI calls because an invalid QtModelIndex does not deliver any information other than being an invalid index (mostly when an item does not have parent aka. invalid parent).

This can be avoided by sharing a reference to a static invalid QtModelIndex to avoid extra JNI calls.