add-type in clipboard-client%

Adds a new data format name to the list supported by the clipboard client.

• (send a-clipboard-client add-type format) -> void
     format : string

  The format string is typically four capital letters. (Under Mac OS X, only four characters for format are ever used.) For example, "TEXT" is the name of the UTF-8-encoded string format. New format names can be used to communicate application- and platform-specific data formats.