Fix changing puzzle size in a maximised GTK3 window.

Fix changing puzzle size in a maximised GTK3 window.

While working on the Net scalability today I noticed that changing
preset from (say) 13x11 to 5x5 in GTK3 Net while the window is
maximised does not have the desired effect (that being that, since the
maximised window does not change size, the new puzzle size is instead
scaled to fit neatly in the existing window).

A git bisect suggests that this was a side effect of commit 8dfe5cec3;
it looks as if there was a useful side effect of setting fe->area as
the 'geometry widget' for fe->window, namely, that any attempt to
resize the window thereafter (even if it had no effect on the window
size) would trigger a configure event on the geometry widget, so we'd
get a notification of our new size even if it was the same as our old
size.

But that 'geometry widget' feature is deprecated, so I have to work
around it another way. Fortunately, I've found a fallback event that
still does occur, namely "size_allocate" on fe->window. So I'm
trapping that as well and using it as an indication that a configure
event won't be forthcoming.