Fixing callbacks and adding Callbacks.md

src/PythonicAPI.md

src/PythonicAPI/Animation/AnimateActors.py

src/PythonicAPI/Interaction/CallBack.md

src/PythonicAPI/Interaction/CallBack.py

src/PythonicAPI/PolyData/PolygonalSurfacePointPlacer.py

src/PythonicAPI/Snippets.md

src/PythonicAPI/Snippets/Callbacks.md

src/PythonicAPI/Snippets/CameraPosition.md

src/PythonicAPI/Visualization/MovableAxes.py

src/PythonicAPI/Widgets/AffineWidget.py

src/PythonicAPI/Widgets/ImageTracerWidgetInsideContour.py