Renaming Slider to Slider3D, adding Slider2D and Slider3D to Snippets

src/Cxx.md

src/Cxx/Widgets/Slider2D.md

src/Cxx/Widgets/Slider3D.cxx

src/Cxx/Widgets/Slider3D.md

src/PythonicAPI.md

src/PythonicAPI/Snippets.md

src/PythonicAPI/Snippets/Slider2D.md

src/PythonicAPI/Snippets/Slider3D.md

src/PythonicAPI/Widgets/Slider2D.md

src/PythonicAPI/Widgets/Slider2D.py

src/PythonicAPI/Widgets/Slider3D.md

src/PythonicAPI/Widgets/Slider3D.py

src/Testing/Baseline/Cxx/Widgets/TestSlider3D.png

src/Testing/Baseline/PythonicAPI/Widgets/TestSlider2D.png

src/Testing/Baseline/PythonicAPI/Widgets/TestSlider3D.png