Property-attribute sync scheme

[nizniz187]

Major changes of WComponent including:

• Refactor default values to attributes object to encompass name, default value, observed, and parser function for easier dynamical parsing.
• Add getObservedAttributes class function for getting observed attributes array.
• Add createGettersAndSetters function for dynamically add getters & setters for all observed attributes.

For more details, please refer to the code comments.
To test if this scheme is valid, please use w-checkbox & w-radio only for testing. (Other component will need to be revised to align with the new scheme to work.)

[nizniz187]

@cwpeng
Please review it first, then I'll fix all the broken components and do the merge.

[cwpeng]

Should we write updating code depending on arguments of attributeChangedCallback, not previous set property?
attributeChangedCallback(name, oldValue, newValue){}

I cannot make setAttribute("checked", false) work after setting it to true.

[nizniz187]

True.
Fixed.
This might be the reason why my demo failed yesterday XD

[cwpeng]

OK.