QtModelIndex performance measurements

Why?

• As a developer I want best performance. There are many cases that the user deals only with an invalid QModelIndex and represents the root index. Extra jni calls can be avoided by adding reusable Q(t)ModelIndex instance.

What?

• Share a reference to a static invalid QtModelIndex to avoid extra JNI calls between jvm & native. To eliminate an invalid QtModelndex going back and forth between JVM & native, 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).