LibWeb: Allow selecting select options without value

[bplaat]

Currently the <select> dropdown ipc uses the option value attr to find which option is selected. This won't work when options don't have values or when multiple options have the same value. Also the SelectItem contained so weird recursive structures that are imposible to create with HTML. So I refactored SelectItem as a variant, and gave the options a unique id. That id is send back to HTMLSelectElement so it can find out exactly which option element is selected.

So now you can select the items in this select for example:

<select>
    <option value="">Lorem ipsum dolor sit amet.</option>
    <option value="">Beatae minus sunt quae dolor.</option>
    <option value="">Inventore nulla doloremque corrupti in?</option>
    <option value="">Laudantium, error. Beatae, itaque sit!</option>
    <option value="">Adipisci a harum dolores molestiae!</option>
</select>

I also updated the styling of a <option> in a <optgroup> a bit by giving the label some indent like other browsers do. And I fixed a bug where the disabled attr was ignored.