FormDialog: Several HIG3 text entry enhancements.

Text entries in forms have been enhanced with basic features:
  - The specific PrivateText/PrivateTextSubmitter code has been removed.

  We now have only one Text/TextSubmitter object allowing different
  types of text to be entered: standard text, password, URI and phone
  number. Each Text/TextSubmitter allows empty values or not.

  - The GtkDialog will disallow validation if basic regexes on one of
    its various TextSubmitters does not validate. It is for example
    impossible to enter invalid URIs. Regexes are only basic checks.
    More advanced checks can be achieved after the Form has been
    submitted by the core components using it.

  - GtkEntries now have an edit-clear-symbolic icon (when not empty)
    allowing the delete the full content at once.