Don't double-bind pointer and mouse events

Prior to this change, using `Blockly.bindEvent_` to bind mouse events results in the callback being fired twice for each mouse down/move/up:

![screen shot 2014-11-13 at 12 31 23 pm](https://cloud.githubusercontent.com/assets/413693/5036025/0eaa4732-6b31-11e4-8a5c-51cb0accc0b4.png)

This was causing parameters to be added twice in the modal function editor, and blocks to be double-disposed when the editor was closed.