Add 2d unifrom scale

User usually expect "shift" key to be the modify key for unifrom scale:

![](http://i57.tinypic.com/2cwpok1.jpg)
A sprite to sclae

![](http://i61.tinypic.com/1zn6i3c.jpg)
Hold "Shift" and dragging corner to scale it uniformly

Godot's old "shift" symmetric scale behavior is keeped by using "Alt" key:

![](http://i58.tinypic.com/2is8mjo.jpg)
Hold "Shift" + "Alt" and dragging corner to scale it uniformly from center