More Forms/Virtual Columns fixes

Hopefully these commits cover all of the permutations...

With these commits applied:
- Virtual columns (including read-only virtual columns) are displayed in forms if the column has an explicit getter or setter in the column config, or has an associated getter or setter in the Model.
- Virtual columns are editable by default if the column has an explicit setter in the column config or has an associated setter in the Model.

Note that AttrConfig.set_read_only! was previously calling virtual?, but virtual? was not actually defined, so that always returned nil.  I've removed the bogus call, since responded_to_by_model? does the right thing with virtual columns anyway.