clarify block descriptions

* `forever` becomes `repeat forever`
* `clear` becomes `clear pen trails`

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I think that `repeat forever` is much clearer than `forever`.
There are three repeat blocks grouped together: `repeat`, `repeat until` and `forever`. `forever` however breaks this group, I think it should be called `repeat forever`. In some translation files, for example in French, this is already the case. Students do not understand instantly how `forever` works, but with `repeat` and `repeat until` they do.

The same thing applies to `clear`. `clear` is sometimes translated as `clear pen trails` - you can take Portuguese as an example - and that is easier to understand, too. I have been asked a lot on how to remove the pen trails.