Add move_forwards(dist) for Actor. Closes #193

[nbogie]

Adds the ability to say myActor.move_forwards(10).

Hi! I hope you don't mind me making a PR without first discussing the feature request. No worries at all if you just close this, but I'd certainly be interested in your thinking on what pygamezero actors should and should not directly provide.

[lordmauve]

This is pretty good.

The problem is that Actors don't have a "forwards" exactly. In the docs we talk about a ship sprite facing right in order to use angle_to() to aim. But that's only a convention. If you have a sprite that faces upwards, you can still rotate it by increasing and decreasing angle.

I would be happy to accept this if you can add docs stating what exactly forwards means.