Couple of solving-related mid-end tweaks. Firstly, when we generate

Couple of solving-related mid-end tweaks. Firstly, when we generate
a game which comes with an aux string, we immediately self-test that
string by passing it to solve() and test by assertion that it
succeeded. So a bug in a back end which intermittently generates
malformed aux strings will be detected as soon as it occurs, instead
of only if the user happens to use the Solve operation on a
particular game in which it happened.

Secondly, Ctrl-S now (undocumentedly) triggers the Solve operation,
on the general principle that keyboard shortcuts tend to come in
handy, and on the specific principle that if you want to look at
lots of solved grids in quick succession (say, when observing their
general shape and nature to see if your generation algorithm was
good or not) it's handy to have a quick way of getting to them.

[originally from svn r8298]