Document usage of qtestlib together with QML in qtestlib-manual.html

Unit tests on a QML component can be done using the qmlviewer application see

• qmlviewer's -script,-scriptopts and -help options
• http://scm.dev.nokia.troll.no/projects/qt/repos/qt/trees/4.7/tests/auto/declarative/qmlvisual/
• tst_qmlvisual.cpp

It might be useful to keep in mind that it is possible have a qtestlib
based unit test for a QML scene. This makes it possible to assess the code
coverage of the test as QML code exercises the underlying C++ API's, or to interact with the objects from C++ to verify logical behavior.

Also it might be worth to link to https://github.com/fgrehm/qmlunit