Ripping shapes

[rupl]

Long, long ago, @afragon suggested the ability to rip a shape into two pieces when grabbed by two people at the same time. Right now it actually displays this effect part-way, but we can detect these events on the server and perhaps clone a shape so that both people have new ones, and the old original is deleted.

To so this we'd make the following changes to socket events:

• [ ] add shape-delete event
• [ ] add shape-clone event
• [x] edit add event (rename to shape-add and create an effect property) fb9c620

The effect flag specifies an effect when the shape appears. Currently it's hardcoded to kind of pop into place. We can set that to default, but allow the creation of shapes with no effect. That will let us control the visual effect of deleting and re-creating shapes on other clients when events like shape ripping happen.

[rupl]

We actually did document this in the beginning: #13