Doc: Fix issues in Qt Help documentation

Fix following issues:
    - Outdated usage instructions on index page
    - Missing C++ class inheritance information
      caused by missing dependency to qtwidgets
    - Broken \snippet in 'Generating Qt Help'

Task-number: QTBUG-34897
Change-Id: I53dd0fbf41e1dfcd41e11f1ab19db7069839477a
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>