Pathgrid loading fix

In pathgrid records, the mCell string is set even for exterior cells, so we can not easily distinguish between exterior or interior cells. To reliably look up the pathgrid for a cell we also need the name and region name of the cell. This fixes the pathgrid for the (0,0) exterior cell not being loaded correctly.

As usual, not sure if the fix should be applied to OpenCS or if that part is even implemented yet, you know it better than me. :)

It should be noted that this system is pretty dumb; e.g. changing the name of a cell or region in a plugin will cause pathgrid edits for that cell to break. I don't think we can do anything about it, since we have to work with the given data model. Future versions of the file format for OpenCS will likely use CellIds, which should not suffer from this problem.