Added "action buttons" to data grid.

Each data grid is going to have at least a few global actions that can
be performed on all of its data -exporting to some tabular format, for
instance.  To that end, I've added an action button bar to the grid that
is simply not visible if no actions are specified.

I've also got support for exporting the data grid to .csv about 50%
done.  The remaining bit should be pretty straightforward - just need to
bang the code out.

Anyway, time to go home and have a beer.