Add pan mode to 2d editor

- Add "Pan" button in 2d editor toolbar, used to toggle "Pan" mode; It's the same "Pan" functionality as middle mouse drag, but with it been toggle on, you can use left mouse drag
![](http://oi61.tinypic.com/1zbdoic.jpg)

- My middle finger get tired easily, and I don't wanna hold on spacebar key all the time, so...

- I draw the hand icon base on shape of my hand, and it's MIT licensed. ( the icon, not my hand )