Get tabIndex, disabled, and onchange working for Slider, and add several automated tests, plus some whitespace cleanup.
Fixes for 2.0 event handling.

1. There shouldn't be any widget methods like onClick() anymore, unless the widget is using it
   internally, but then it should be called _onClick() with an underscore.
2. Widgets already inherit methods like onclick and onchange from HTMLElement,
   but they should define functions like onfoo() for non-standard event names.
3. All listener functions (even onchange) get called with single argument: the Event object.
Fixes and test for parsing attributes during declarative instantiation:

1. Allow globals (ex: store="myStore") for Object attributes.
2. Fix functions attributes like funcProp="console.log(...)"; they were being ignored.
3. Disallow globals for functions, ex: onclick="myFunc", since that's inconsistent
   with how native DOMNodes work, where you need to do onclick="myFunc();"