Let right mouse button to be activation key

Adds RMB to the list of activation keys. Because TZP is usually activated by mouse, having one of the mouse buttons in activation key role is little bit more comfortable.

Tested on Windows, Firefox 15.0-32.0a1. Requires Firefox 15.0 or later. It won't work on Mac OS as `MouseEvent.buttons` is not available there.

If you like it I could probably make it version and platform aware, so it would work for Firefox >=15.0 on Windows or Linux.