Add interface for get cropped geometry from VisualizerWithEditing class

[yuecideng]

1. Add a function get_cropped_geometry to get the final cropped geometry from VisualizerWithEditing.
2. Seperate saving cropping results from key press C to another key press S.
3. The current workflow maybe like this:

vis = o3d.visualization.VisualizerWithEditing()
vis.create_window()
# pointcloud or trianglemesh
vis.add_geometry(geometry)
vis.run()
# do cropping once or multiple times, press key `C` to crop selected geometry and `S` to save it into file.
vis.destroy_window()
# get cropped result to do somthing else
cropped_geometry= vis.get_cropped_geometry()

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