Fork the quicknanobrowser as tests/quicktestbrowser

This also removed experimental API uses from the quicknanobrowser example,
which should be used mainly for documentation purposes.

The quicktestbrowser should be the one that we use from now on as a raw testbed
of new APIs.

As with other targets in the tests directory, it will only be built by default
if Qt is configured with -developer-build (and without -nomake tests).

Change-Id: Ib4461c898cd3227bbb810493daac4d841d0d8f3e
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>