Updates the Spring tester app

What it does
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I had a lot of fun using your spring code in a project, but I had a hard time find the "right" values for my application. So, I modified your demo app to use sliders instead of text boxes, and I made it automatically animate as you change values. This way, you can test out different combinations of values in real time.

I also added another text field at the bottom for copying these values into your code. It lets you name a variable (as it appears in whatever app you're using the spring), and then generates the code according to the sliders you've set up. It made testing out springs much, much easier.