Use dhtmlxlayout to implement the split in dhtmlxgrid

Dhtmlxgrid’s split implementation is not very user friendly in terms of how the user drags the splitter (which is really just one side of a column header on right split). It would be much better if there was a true splitter bar that is independent of the headers, and it ran all the way down the grid, so the user could grab it anywhere in the grid.

This seems like it could easily be implemented with dhtmlxLayout…one split grid on the left pane and one split on the right.