Implement :enabled & :disabled CSS selectors

Specs:
http://www.whatwg.org/specs/web-apps/current-work/multipage/selectors.html#selector-enabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/selectors.html#selector-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/common-idioms.html#concept-element-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#concept-fe-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#concept-option-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#attr-optgroup-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#attr-fieldset-disabled
http://www.whatwg.org/specs/web-apps/current-work/multipage/interactive-elements.html#attr-menuitem-disabled

HTMLMenuItemElement to be implemented in #2673.