Control subclass for button

[sjjhsjjh]

The ControlPanel piece, in the controlpanel.js file, has a private class, Control, to represent an individual control like a button, select, or text input. At time of writing, the Control class code has:

• A _construct_button() method.
• Checks like $.control === "button" in a few places.

There should instead be a subclass like Button that extends Control.

The code that instantiates controls, look for "new Control", should check the $.control value and instantiate the Button class instead.