Feature/stream tracer widgets

See merge request !39

The underlying vtkImageMapToColors contained in the vtkImagePlaneWidget
only considers the active scalar of the input.

The trick is to use a shallow copy of the input image to set the active
scalar on its fields without corrupt other decorators.

Paraview has not such a problem since it does not use the
vtkImagePlaneWidget.

Simplifications have also been made to avoid spurious calls to set the
positions of the slices.

Need to be tested using sdm data. Especially for the slices and the
volume.

Bugfix: changes in the widget controls would move the seed, but would not update the streams. This is now fixed.

Simpler implementation : the reset event is handled internally by our class, no need to connect to another signal by the caller.

done automatically by the WidgetsCategory constructor.

We store the shared fields in the parent class DecoratorWithClut, and remove them from the inheritors.
These fields are: "Display", "Scalar Bar", "Colors", "Fixed Range", "Component".

But it is still unstable.