Adding a stub Clock class and providing a fixed update event in Window and View

[DragonMoffon]

Changes:

• clock.py with a Clock and FixedClock object
• on_fixed_update in Window and View
• deleted old experimental clock and window
• an example showing of value interpolation for smoother fixed updates

[einarf]

I think this is pretty good and other libraries / engines do this. Possibly we need to get control over all the properties attached to the view and window.