Move the disabled property to specific interfaces

[BastienClement]

PR #193 changed the disabled property on HTMLElement from Boolean to js.UndefOr[Boolean].

Actually, there is no disabled property defined on the HTMLElement interface:
https://html.spec.whatwg.org/multipage/dom.html#htmlelement

This property is instead defined on specific sub-interfaces. So far I found 9 of them in the one-page specification for HTML:

• HTMLInputElement : https://html.spec.whatwg.org/#htmlinputelement
• HTMLButtonElement : https://html.spec.whatwg.org/#htmlbuttonelement
• HTMLSelectElement : https://html.spec.whatwg.org/#htmlselectelement
• HTMLOptGroupElement : https://html.spec.whatwg.org/#htmloptgroupelement
• HTMLOptionElement : https://html.spec.whatwg.org/#htmloptionelement
• HTMLTextAreaElement : https://html.spec.whatwg.org/#htmltextareaelement
• HTMLKeygenElement : https://html.spec.whatwg.org/#htmlkeygenelement (which is being removed from the HTML spec and not included in Scala.js DOM)
• HTMLFieldSetElement : https://html.spec.whatwg.org/#htmlfieldsetelement
• HTMLMenuItemElement : https://html.spec.whatwg.org/#htmlmenuitemelement (which does not exists in Scala.js DOM)

This pull requests moves the disabled property from HTMLElement to the 7 sub-interfaces already present and reverts its type to Boolean.

I stumbled upon this while trying to define a custom element (extending HTMLElement) that required a custom disabled property. The compiler is falsely complaining that "variable disabled cannot override a mutable variable" even no such variable actually exists.

[sjrd]

LGTM, Thanks!

Sorry for the delay, I was on vacation.