Cleanup and refactor the main CSSStyleDeclaration class

* Stop making underscored ("private") properties and methods non-enumerable. We will eventually use jsdom's wrapper architecture to hide them instead.
* Group the private properties into logical categories and make them closer to what the spec uses.
* Remove the length setter.
* Simplify indexed property handling and remove dead code. In the future jsdom's wrapper architecture will take care of many details here.
* Rename arguments for clarity, notably "prior" to "priority".
* Add jsdoc.

Author: asamuzaK