Missing Element.remove

[OndrejSpanel]

It seems DOM Element (in org.scalajs.dom.raw) misses the remove method.

It can be replaced by e.parentNode.removeChild(e), but I think there is probably no reason not to add it.

[strelec]

Many more elementary DOM operations are missing (all of them very well supported in the browsers):

	@js.native
	trait ElementExt extends HTMLElement {
		def remove(): Unit = js.native
		def before(nodes: Node*): Unit = js.native
		def after(nodes: Node*): Unit = js.native

		def replaceChildren(nodes: Node*): Unit = js.native
		def prepend(nodes: Node*): Unit = js.native
		def append(nodes: Node*): Unit = js.native
	}

[armanbilge]

@strelec would you mind opening a PR with those additions please? Would be much much appreciated!