Added a ProgressBar widget, essentially a non-interactive Slider

Created a ProgressBar widget that behaves and looks like a non-interactive Slider, which in fact it is. All the source code from Slider class has been brought over here, except the user interaction part, which was intentionally left in the Slider class, which now extends the ProgressBar class by adding event listeners and rest of interactivity to the component.

This approach, I believe, is more logical than simply extending existing Slider class and suppressing all event  listeners and user interaction there by overloading constructors and public methods. However, if you believe the Slider class should be kept absolutely intact as is, and ProgressBar added on top of it, I can submit that code change as well.

The reason I am not willing to simply take a Slider and disable its touch listeners (or remove them after creation) as suggested in libGDX wiki is the desire to keep the widget logic clean and separate.

As usual, I am open to all thoughts, comments, suggestions, and constructive criticism.