4.9.0 readiness

I think now I finished the things I still wanted to do before the 4.9.0 
release:
* Controlling castling rights in Edit Position mode,
* Making the font handling consistent,
* Adding the texi-file description of a dialog I had so far forgotten,
* Updating the what's-new page for these features.

The sizing bug reported by Adrian Petrescu still stands, but since I did
change a fair amount in that area, I wonder if that problem still exists.