C++ editor: Improve type hierarchy widget

Now the type hierarchy widget will also show the classes
derived from the selected one. For consistency the way
the base classes are shown was changed too. The diagram
below is an example from Creator's code when openining
the type hierarchy for BaseTextEditorWidget:

Bases
 +QObject
  +QWidget
   +...
     BaseTextEditorWidget
 +QPaintDevice
  +...
    BaseTextEditorWidget
Derived
 +BaseTextEditorWidget
  +VCSBaseEditorWidget
    GitEditor
    MercurialEditor
    ...
   GLSLEditorWidget
   CppEditorWidget
   QmlJSTextEditorWidget
   ...

Depending on the project and on the selected class the
hierarchy widget might take a bit to be constructed.
This should be improved later.

Change-Id: Ifbdd1cbbba955a0bdf03297ff0e7620351b12dc5
Reviewed-on: http://codereview.qt.nokia.com/883


Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>