libqqwing: add get_version()

This might well be my last pull request, at least for a while. We have a branch of GNOME Sudoku that uses qqwing to generate puzzles, and it works great! I also got final approval for GNOME to depend on qqwing. I will merge our qqwing branch as soon as you release a new version, since we need the getPuzzle() function I added a couple of days ago.

Anyway this request is not needed, but a nice-to-have: we'd like to display "Puzzles generated by QQwing 1.2.3" on our about dialog. (qqwing is such a major dependency, it deserves a little bit of advertisement, I think. :)