Reload on save

Originally reported by: **Daniel Pope (Bitbucket: [lordmauve](https://bitbucket.org/lordmauve), GitHub: [lordmauve](https://github.com/lordmauve))**

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Rather than requiring users to close and restart the game, we could do this automatically when the game file is saved.

This would remove a source of confusion as well as make the development experience more interactive.

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- Bitbucket: https://bitbucket.org/lordmauve/pgzero/issue/44

The issue names no implementation files or tests. Start by locating how the game file is loaded and how the running game is closed or restarted; define the save-detection and reload behavior before implementing. Done means saving the game file reloads it without requiring a manual restart, with coverage for the supported workflow.

Written by the indexing model from the issue text.