Add Qt JNI API.

The QJNI API's provide an easy way to communicate with Java API's from
C++. The API consists of two classes, QJNIObject which acts as a wrapper
around Java classes and QJNIEnvironment which attaches the current
thread to the Java VM and exposes the Java Native Interface.

Change-Id: I9cdc395249de74f940b75eab9f2bd32347d5d432
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>