Change vgrid setup to be sparse?

[chrispsn]

Make vgrid an object that just holds cells, maybe indexed by row-col string pairs? (eg '3-1')

Then grid_component can take that and render it and fill in the gaps.

selected_cell can be an address (again, eg '3-1') in state.

Methods which would have previously hit an empty cell would instead catch that and refer to EMPTY_CELL.

Maybe the Sheet object can abstract this level of detail.

And we can get rid of the EXTEND_GRID action (and associated calls).

JavaScript array details: http://stackoverflow.com/a/1510842

[chrispsn]

Done, but I wonder whether the creation of cell IDs should be pushed into the cell creation process, rather than the grid component.