[Gzz] 30th-31th (Benja)

[B. Fallenstein]

I've committed a change to ScrollSpanMaker that creates new 
TransientTextScrolls when the old ones have gone immutable.

REASON: When saving one of its spans, we can simply finalize the 
TransientTextScroll (then save it, then use its id). We don't need to make it 
create new blocks internally, nor notify its SpanMaker. The SpanMaker simply 
makes intelligent use of the ImmutableExceptions, for a change.

DOWNSIDE: ScrollSpanMaker now uses only TransientTextScrolls, no other kind 
of text scroll. This is not a problem IMO-- you want something else, you just 
create a different SpanMaker. It's less effort than creating the new 
TextScroll implementation.


Here's one other thing to consider, for sometime--
    What we really want to do is making blocks not with all spans, but with 
only those that our SliceVersion references. Then we don't have the 
difficulty with FakeSpans: When we save and load again, the fake spans must 
still be there-- so we do need to save  them somehow-- and if they're copied 
to 'real' user cells, they must acquire global ids-- yet when we create a new 
one each time the cursor moves, say, we have far too much data. So we simply 
save those that are there when we save a version-- that is reasonably cheap...

But as I said, that's not for now.
-b.