all var sheet dialogs are GObjects; master merged into psppsheet

[Ben Pfaff]

This afternoon, I completed the conversion of the dialog boxes in
the variable sheet from ad-hoc structures into proper
GObject-based dialog boxes.  Then I merged "master" into the
"psppsheet" branch.

I wanted to send out these changes for review, but I especially
wanted to do the merge after the conversions, which required
having the conversion done.  Please, if you notice any bugs,
report them and I will fix them.

If I have more time today then I will work on finding and fixing
memory leaks, as John requested a few days ago.

Thanks,

Ben.