Covering case when a test column contains more enum values
than the corresponding column in test data.
Proper deletion of adapted frame.

The deletion is tricky - the code records all newly created
vectors into a dedicated frame and delete it.
Dummy fix of columns field description.

This is not conceptual change since it propagates help string
only for one particular UI element.
Fix of source input field bug.

The source input field was missing a check for required flag.
So any name of key was able to be passed as a valid key even it did not
exist.