Improve display for sold out price set options

If a price set option is sold out the display does not provide enough information to the user that it is 'sold out'.

It behaves differently depending on the price field 'input field type':

Text/Numeric quantity (ie text input box) - The input box just disappears completely.

Select - No information until you try and book, then a helpful message appears to say that it is 'Sold out'.

Radio - Replaces the radio icon with a '( )' (frozen quickform element).

Checkbox - Replaces the checkbox with a '[ ]' (frozen quickform element).

This is demonstrated here:
http://drupal.demo.civicrm.org/civicrm/event/register?cid=0&reset=1&id=7

I propose that that in all cases the input field is replaced with 'Sold out' (with a span around it so that it can be easily themed as desired).

In the case of the select list, the option should be prepended with 'Sold out'.

Thanks to bgm for help in getting to the bottom of this. Line 323 in Price\BAO\PriceField.php freezes the elements, so there's a starting point. I'll try to put together a pull request for this.