[ticket/13055] Fix string profile fields validation to support unicode

Add unicode support to the string type based profile fields.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13055">PHPBB3-13055</a>.
As per the language entries, ALPHA_ONLY stands for 'Alphanumeric only', and 'FIELD_INVALID_CHARS_ALPHA_ONLY' states that 'The field “%s” has invalid characters, only alphanumeric characters are allowed.',
So I've included numbers here as well.