Time to merge rewrite-sheet

[John Darrington]

I'm beggining to think that the rewrite-sheet branch of the git
repository is now stable enough that it ought to be merged into
"master".

Benefits it offers include:

* It's about 4000 lines less code.

* Redrawing the sheets no longer flickers and is much smoother.

* When dealing with large numbers of variables scrolling is much faster.

* There's the "split window" menu item, which allows you to see both
  ends of the datasheet together.

* One can individually adjust the height of rows.

The only major functionality that's currently missing, is the ability
to select cells for cut-n-paste.  I'm going to try to fix that
shortcoming in the next few weeks. But if people can live with it,
then I think merging the branch and closing it off will be the best
way to discover all the bugs.

A couple of other points:

* I've abandoned any attempt to keep GtkSheet compatible with the
  original GtkSheet.  It now bears little resemblance at all.  So I
  think it'll need to be renamed, and moved to a new directory.

* Since I'm now using Ben's tower structure from libpspp/tower.[ch]
  which is GPLv3, the question over licensing has been resolved; it
  must also be GPLv3.

Any comments?


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