[Form] Use an horizontal rule to separate preferred choices

Today, we use by default the following text-based separator for preferred choices in ChoiceType:

Line 243 in fafbbfa

'separator' => '-------------------',

Some browsers just added support for adding <hr> as separators inside <select> elements, which looks much better. See https://developer.chrome.com/blog/hr-in-select/

If this proposal is accepted, it can only be implemented in a few months, when most browsers support this feature. Thanks!

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