HTMLSelectElement doesn't support `selectedOptions` property

[AndrewKushnir]

The selectedOptions property is not supported on the select elements in Domino based on the definition here:

domino/lib/htmlelts.js

Lines 959 to 979 in 12a5f67

	define({
	tag: 'select',
	ctor: function HTMLSelectElement(doc, localName, prefix) {
	HTMLFormElement.call(this, doc, localName, prefix);
	},
	props: {
	form: formAssociatedProps.form,
	options: { get: function() {
	return this.getElementsByTagName('option');
	}}
	},
	attributes: {
	autocomplete: String, // It's complicated
	name: String,
	disabled: Boolean,
	autofocus: Boolean,
	multiple: Boolean,
	required: Boolean,
	size: {type: "unsigned long", default: 0}
	}
	});

The selectedOptions property is generally supported in all modern browsers and it'd be great to have a support in Domino as well. An absence of this support requires including extra code (like this one) into app bundles, thus increasing it unnecessarily.