XBoard 4.9.0 release

I did sync with Savannah, and push everything I had in my master branch 
to it.
I don't know how topush my 'themes' branch, which does not exist yet at 
Savannah.
Anyway, I think we are as ready for 4.9.0 as we will ever be. So perhaps 
we should
just release what we have.

A minor issue is whether we want to continue providing oriental-style 
Xiangqi and
Shogi piece themes in the standard XBoard distribution tar ball, now 
that we have
(more elaborate) tar balls also containing these same files. If we 
don't, we should
remove them from the XBoard Makefile.am, and from the 'themes' directory
(so they only remain in the 'themes' directory of the 'themes' branch). 
We should
post the distribution tar balls of theses themes on Savannah somewhere 
before
we do that, though.