Erasing functionality first prototype

This is a very simple implementation of the erasing functionality as originally specified in our team agreement - I have gathered improved ideas from that implementation but it is still good as a proof of concept.

As we can only erase erase points that were drawn, the quality of erasing depends on the number of points drawn initially (i.e. the speed at which they were drawn).

Lastly, we are reaching the WebRTC message limits (need to investigate that further later) because of the ridiculous message overhead from yjs (as the library is far too general than what we need) - therefore, if you draw and delete too much, new clients won't be able to sync as the state cannot be fit in a single message.

@gc4117 @mnr17 @im2517 Please have a look and review

@mnr17 Please merge this merge request before !25 (merged) as there was a lot of restructuring of the drawing code