Add 3D cursor widget

Add a new widget representing a 3D cursor. This cursor is intended to be used in a stereo environment. It takes form of an actor (a cross by default) that is added to the scene, hence it's displayed in stereo like other actors. We take advantage of the existing API for widget handles in vtkAbstractWidget to insure the cursor keeps the same size on the display.

Previous POC has been done using an interactor style switch instead a widget (see !9715 (closed)) but had been dropped in favor of this one (mainly because a lot of needed functionalities already exists in vtkAbstractWidget).

Current limitations:

• Do not work with volumes (for now no pickers handles them properly)
• Unsteady placement on other widgets (manipulation and cursor actualization remain fine)
• When zooming the cursor do not follows the mouse until moving it again (a workaround was done in the interactor style switch approach, but it require to manage multiple callbacks priority, abstract widgets only having one "big" callback using the callback mapper)