This commit fixes warning like one reported below occuring when imlementing
custom event players:

```
In file included from /home/jcfr/Projects/Slicer/Libs/MRML/Widgets/qMRMLCheckableNodeComboBoxEventPlayer.h:25:0,
                 from /home/jcfr/Projects/Slicer/Base/QTGUI/qSlicerApplication.cxx:82:
/home/jcfr/Projects/Slicer-Release/CTK-build/QtTesting/pqWidgetEventPlayer.h:60:16: warning: ‘virtual bool pqWidgetEventPlayer::playEvent(QObject*, const QString&, const QString&, bool&)’ was hidden [-Woverloaded-virtual]
   virtual bool playEvent(
                ^
In file included from /home/jcfr/Projects/Slicer-Release/CTK-build/QtTesting/pqTreeViewEventPlayer.h:35:0,
                 from /home/jcfr/Projects/Slicer/Libs/MRML/Widgets/qMRMLTreeViewEventPlayer.h:25,
                 from /home/jcfr/Projects/Slicer/Base/QTGUI/qSlicerApplication.cxx:85:
/home/jcfr/Projects/Slicer-Release/CTK-build/QtTesting/pqAbstractItemViewEventPlayerBase.h:51:8: warning:   by ‘virtual bool pqAbstractItemViewEventPlayerBase::playEvent(QObject*, const QString&, const QString&, int, bool&)’ [-Woverloaded-virtual]
   bool playEvent(QObject* object, const QString& command,
        ^
```