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();"