Early initialization and fix errors from vtkOpenGLExtensionManager

I worked a bit more on a ![QML prototype for MITK](https://github.com/maleike/MITK/tree/personal/maleike/qml-openview-rendering/Examples/QuickRender) and made three minor changes/improvements to QVTKQuickItem which I hope you find useful and are able to integrate:

 - vtkOpenGLExtensionManager errors of type "Attempting to load GL_VERSION_2_0, which is not supported." could be removed by a Qt::DirectConnection to IsCurrent()
 - introduced a counter-part to prepareForRender() called cleanupAfterRender()
 - the initialization that is being done once in the paint() method has been moved to the c'tor with the intention that sub-classes may access the VTK render window in their c'tor already. This makes using the class much easier in the MITK application mentioned above

Behavior the OpenView application did not change as much as I can see.