This grid code is so bad already so why not make it a bit worse? Make most of...

This grid code is so bad already so why not make it a bit worse? Make most of the private fields in BaseGrid protected instead so SubsGrid can also access them. It's not like there is a clean distinction between the responsibilities of those two anyway. This lets us reset extendRow after deleting lines in some SubsGrid code. This fixes #1348.

Originally committed to SVN as r5765.