Change of context

Originally reported by: **Tim Golden (Bitbucket: [TJG](https://bitbucket.org/TJG), GitHub: [TJG](https://github.com/TJG))**

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We could have a mechanism to change "levels", ie to swap out/in a set of draw/update & handlers. My idea would be to have a series of modules so you could activate() one module or another.

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- Bitbucket: https://bitbucket.org/lordmauve/pgzero/issue/17

Start by reviewing how the framework currently handles draw/update functions and handlers, then determine where levels or modules could be activated. The issue needs a settled design for swapping these sets and a clear example or test showing that changing levels selects the intended handlers.

Written by the indexing model from the issue text.