Implement CSS selectors :enabled & :disabled (incl. HTML Elements support)

Specs:
http://www.whatwg.org/html/#selector-enabled
http://www.whatwg.org/html/#selector-disabled
http://www.whatwg.org/html/#concept-element-disabled
http://www.whatwg.org/html/#concept-fe-disabled
http://www.whatwg.org/html/#concept-option-disabled
http://www.whatwg.org/html/#attr-optgroup-disabled
http://www.whatwg.org/html/#attr-fieldset-disabled
http://www.whatwg.org/html/#attr-menuitem-disabled

HTMLMenuItemElement to be implemented in #2673.