Allow connected clients to share mouse position

You can click on somebody (including yourself) in the sidebar to toggle mouse sharing on and off. It picks a color based on their randomly generated client id.

Looks like this (the red mouse position on the left side is added by the screengrab software):

![p242ecfzt5](https://cloud.githubusercontent.com/assets/476818/4803902/431c3fa8-5e60-11e4-9103-34cdcbf7e16a.gif)