Bug #729084 - Use tel: URI instead of callto: for phone numbers

This change does a bit more, actually:
- learn the code of "tel" URI, being handler similar to "sip", "callto", "h323"
- generate tel: URIs for phone numbers, but only if the leading character of the
  phone number is a plus ("+") sign
- always remove spaces in href part in the e_text_to_html() when
  E_TEXT_TO_HTML_URL_IS_WHOLE_TEXT flag is set