Allow Actors to move forwards an amount according to their angle

[nbogie]

It'd be very useful to be able to say myActor.move_forwards(10) now that actors have a rotation.

It's quite a barrier for some kids just starting out in python to be faced with the trig before they can get such movement working - especially when it was so simple for them in Scratch.

[ericclack]

+1 from me. We're planning on introducing PyGameZero at Brighton Coder Dojo and anything that makes the step from Scratch easier would be great.

[QuirkyCort]

Wrote a helper library that adds in some of the capabilities in Scratch that are missing in Pygame Zero (eg. move forward, flip, scale, pixel perfect collision) plus a few more (eg. fullscreen, hide mouse). You can find it here https://www.aposteriori.com.sg/pygame-zero-helper/

This helper library should eventually be obsolete when these functions are added to Pygame Zero itself.