rqt_graph: changed the method of zooming

This new method handles smaller step sizes. The trackpad on Macbook Pros have a step size which ranges from +/-2 up to +/-1200, but with a normal mouse, in OS X or Linux, the step size is always +/-120 or +/-240. This new method scales better with smaller step sizes, making scrolling smoother when using a track pad, which was pretty much useless before.