Make ctkIconEnginePlugin derive from QIconEngineV2 for Qt 4

Qt5 removed QIconEngineV2 all together. However, for Qt4,
The icon engine must derive from QIconEngineV2 in order to be able
to stream icons (used when drag&drop in QStandardItems).
Fix ctkIconEnginePlugin moc-ing with Qt 4

Qt4 moc parser does not expand macros such as QT_VERSION_CHECK
Merge branch 'fix-ctkIconEnginePlugin-for-qt4'

* fix-ctkIconEnginePlugin-for-qt4:
  Fix ctkIconEnginePlugin moc-ing with Qt 4
  Make ctkIconEnginePlugin derive from QIconEngineV2 for Qt 4