ColorField: switch data type do unicode

* Make ColorField work with unicode by default.
* Make ColorButton accept both str and unicode and make read() work like
  other proxy widgets (where they call self._from_string, making it be
  converted to data_type accordingly).
* Add a FIXME on _from_string and _astring to make them public. All of their
  callsites are outside the class/module itself.

Change-Id: I9c1c0c0b2ff4619153447e122f50b9004f2dc907