Volume Slider & Mute Button

Added in the volume slider and mute button to the Settings screen in the pause menu.

@kadst43 @AmbientOSX @ajiraffe 

The Switch Scenario button in the pause menu doesn't do anything just yet. I'll add that in a separate pull request.
Yes, it should. Unfortunately, when I tried it, I got a really obscure bug where `onmouseover` and `onmouseout` stopped working. I couldn't find any help online, so I changed it back. It wasn't worth the time to try to debug. Hopefully, I can get back to it at some point.