A fix for ProportionalSplitterMorph, take 2

> Repeat:
>
> Attached is a fix for ProportionalSplitterMorph.
>
> Currently, moving a window splitter changes all relevant frame layout
> offsets; fractions are not touched. But offsets are absolute values, in
> pixels, and a frame with a large offset will be badly redimensionned
> when the system window extent is reduced.
>
> To see this, open a browser, and move the main horizontal splitter way
> up. Then shrink the browser: the code pane will keep a largeish height
> (because of its large vertical offsets), and eventually be moved above
> the browser top.
>
> To fix this, I propose in the attached changeset to keep track of the
> overall splitter delta, then on #mouseUp to recompute the fractions and
> reset the offsets to the typical small values they have.
>
> I don't know if I am being clear, but it works... try it.
>
> (the #balanceOffsets method may possibly be simplified I guess)
>
>
> Stef