Internal (Framework): New "Data sourced" matters

* FW_DATA_SOURCED: GUI "widget" objects that know how to move data to
and from their data sources (e.g. model objects)
* FW_TEXT_FIELD: An example of an EV_TEXT_FIELD which knows how to be
FW_DATA_SOURCED.
* TEXT_FIELD_TEST_SET: A series of tests for the FW_TEXT_FIELD and its
ancestor.