Add sprite.hide and sprite.show

[ericclack]

We can delete sprites with sprite.delete() - but hiding and showing would be useful because...

What's the use case?

[ericclack]

In Scratch you cannot create or delete sprites in code (except for clones) so show and hide are essential. However we can create and delete sprites.

[ericclack]

Coders can move the sprite off screen fairly easily by setting negative x and y.