Introducing Smack's own XmlPullParser interface which tries to stay as
compatible as possible to XPP3. The interface is used to either wrap
StAX's XMLStreamReader if Smack is used on Java SE, and XPP3's
XmlPullParser if Smack is used on on Android.
Fixes SMACK-591.
Also introduce JUnit 5 and non-strict javadoc projects.
This also moves the logic to send error IQ replies from "when there is
no IQ provider registerd" to "when there is no IQ request handler
registered". Which has for example the advantage that IQ parsing no
longer asks for a connection instance.