Add more keypress events to VTK visualization module

[orbingol]

The following list is available directly from the default vtkInteractorStyle used:

• e: exit the application
• p: pick object (hover the mouse and then press to pick)
• f: fly to point (click somewhere in the window and press to fly)
• r: reset the camera
• s and w: switch between solid and wireframe modes

It would be nice to add option to change visibility of some other elements, such as control points, objects, etc.

[orbingol]

Just updated the original issue with the list of available vtkInteractorStyle keypress events. I have added the following custom keypress events:

• b: change background color
• l: change color of the picked object
• d: print debug information (of picked object, point, etc.)
• h: change object visibility
• n: reset object visibility
• arrow keys: pan the model

They are documented here: https://nurbs-python.readthedocs.io/en/devmaster/module_vis_vtk.html#geomdl.visualization.VisVTK.VisConfig.keypress_callback