[wxPython] More on custom grids

I wrote before

This comes close to doing the right thing, but needs a manual window
resize to get all the columns to line up correctly. I also tried
deleting all the columns/rows and then appending them all again.

In fact, it only seems to work when the table grows. If my new table
ends up smaller then It seems to get confused, wanting to access
columns that aren't there anymore. BTW it's wxgtk-2.1.15.

Tim