Move newViewRequested to the public API

This moves the API to public, with proper versioning, adds
documentation and adjust the warning to also report an null parameter
to openIn that would cause the load to fail.

The experimental example code is copied from quicktestbrowser to
quicknanobrowser.

Change-Id: I23b06c7a5add0323d0540a783873584438d85ea8
Reviewed-by: Szabolcs David <davidsz@inf.u-szeged.hu>
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>