Fix #1691 (grid): generate column defs even if rowHeader already present

The data watch only generated column defs if there were no columns.  But
if the selection feature was enabled then there would be a rowHeader column
in place, and therefore it wouldn't generate column defs.
Merge pull request #1695 from PaulL1/1691_no_column_defs

Fix #1691 (grid): generate column defs even if rowHeader already present