Movie kind of works.

Until now only red slice works. View must be axial.
Movie works for all three SliceWidgets.

Several changes regarding the UI were necessary to enable the user to choose which SliceWidget should show the Movie.
Changed property "nodeTypes" of OutputCheckerboardComboBox to "vtkMRMLScalarVolumeNode" to fix crashing.
Disable movie button while movie is running.

Multiple klicks on the button would start multiple movies and Slicer could not be closed properly.
Changed line-endings.
Enabled linking of slices (for Flicker).
Flicker works with fixed frequency.