Add __repr__ to built-in objects

Having added a REPL (in #80), Pygame Zero's built-in objects can now be explored in the REPL. However, they do not generally have a defined `__repr__`.

For example,

```python
>>> screen

>>> keyboard

```

Implementing this would make these more usable in the REPL.

Start from the REPL introduced in issue #80 and locate the built-in objects shown in the examples, especially screen and keyboard. Check how their classes are defined and whether tests cover their displayed representations. Done means the built-in objects expose useful __repr__ output instead of default memory-address representations when explored in the REPL.

Written by the indexing model from the issue text.