sheet rework

[John Darrington]

I've just pushed a series of changes to "master", which attends to
a number of issues in the sheet implementation.  In particular, 
selecting cells, which had been broken for some time, should now
be working again (and better this time).

As a bonus, psppire-sheet.c is now below the 5000 line mark!

When doing this kind of work however, it can be difficult to ensure 
that subtle bugs haven't crept in.  If you notice anything wierd
going on, with respect to selection of cells, or the sheet in
general, please post a bug.

J'

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