QmlMitkFourRenderWindowWidget and QmlMitkRenderWindowItem

Attempt to integrate MITK render windows into a QML scene.

This change brings two QtQuick items

• QmlMitkRenderWindowItem as a mitk::RenderWindow variant based on Kitware's OpenView component QVTKQuickItem.
• QmlMitkFourRenderWindowWidget as a replacement for QmitkStdMultiWidget with much reduced functionality

Most things are working nicely now, major issues to solve:

• Qt Quick renders in a second thread, which collides with some things in MITK. This demo brings only a workaround and not a solution.. See QmlMitkBigRenderLock.
• Events handled by QmlMitkRenderWindowItem are probably incomplete
• QmlMitkFourRenderWindowWidget and QmlMitkRenderWindowItem need cleaned-up interfaces to be really usable for applications
• The new modules must be nicely integrated in to the build system
  • find and use Qt5 optionally, make this a condition to QmlMitk
  • bring the minimal changes back to OpenView (pull-request submitted)
  • build OpenView as a package dependency
    • (patch OpenView if neccessary)