LP#1835982: add GridCellTextGenerator to the btGrid in the sandbox

LP#1835982: add GridCellTextGenerator to the btGrid in the sandbox

This patch also adds a usage note: since the GridCellTextGenerator
only has access to the row object but a cellTemplate can be passed
arbitrary context in addition to the row, a GridCellTextGenerator
that needs that additional context to have the print content match
the displayed content may require that the caller stick it in the
row object.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>