Complete test suite for classes in applications/models.

Develop additional automated regression test cases, completing the test suite for the application/models directory:
  - ExternalTableReferenceTest.php
  - SetTableTest.php
  - TVSFactoryTest.php
  - TableViewSequenceTest.php
  - additional basic Field test cases (primarily in BasicFieldWithDBAccessTest.php and FieldWithExtConnAndDBAccessTest.php)

Continue to update docs/TableSettings, although it is still under construction.

Move the activity and table setting files used for regression testing from the application/settings/tests directory structure to tests/application/settings.

In the test case classes, move the specification of most test activity/settings files to constant definitions in TestSettings.php.

Make minor other updates to previously-written test case files.

Make minor updates suggested by testing results (several new error conditions checked, better error messages, improved documentation comments) to the following classes in the application/models directory:
  - Field.php
  - SetTable.php
  - TVSFactory
  - TVSGateway
      - make minor updates, similar to those in other files
      - add new getTopLevelTableName method
      - fix functionality so that files with sequence information but no top level table name are still recognized as valid sequence/setting files

  - TableViewSequence
      - add new static function that enumerates the valid sequence setting properties
      - add new getSeqLevelTableName method

Add support for new selectFrom table setting property, which provides a drop-down menu, similar to that provided for enum types, but using the data from a specified field in an external table as the list of possible valid values.

__New Classes:__
  - application/models/DbTable/ValidValuesSource
  - tests/application/models/DbTableValidValuesSourceTest.php
__Modified Classes:__
  - application/models/Field.php (particularly getValidValues())
  - application/forms/TableRecordEntry.php
Remove reference to unused constant from view script
Modify initFrom fields in ModuleOfferingAdd