QGAL: Refactor solvePreferred to support min/maxPrefSize

With the addition of min and maxPrefSizeHints, the logic used in the
preferred simplex had to be changed. The concept behind it is still
the same, we minimize the deviation from each item's preferred size.

Previously we would make an special effort to avoid items being shrunk,
that's why the "shinker" simplex variables had larger multipliers in
the objective function.

Now we still have variables with large multipliers and others with
small mutlipliers, but rather than differentiating between shrinker
and/or grower variables, we classify them as "soft" or "hard".

"Soft" variables, or slacks, are those that allow the anchor to change
its value up to the minPref or maxPref boundaries, OTOH, "hard" slacks
are those that allow the item to go all the way to its minimum or
maximum sizes. Naturally, the "hard" slacks get large coeficients
in the objective function while the "soft" ones get small ones.

Now all the slack variables are restricted in size as to respect
the boundaries of each interval:

 - Minimum to MinimumPreferred,
 - MinimumPreferred to Preferred,
 - Preferred to MaximumPreferred and
 - MaximumPreferred to Maximum

With such limits, the SizeHint constraints became redundant in the
calculation of preferred sizes so they are no longer used.

Additionally, as an optimization, if a given interval is NULL
(for instance, MinimumPreferred is equal to Preferred), then
the associated slack variable would have its size restricted to
zero, therefore it is not created at all.

Signed-off-by: Eduardo M. Fleury <eduardo.fleury@openbossa.org>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@openbossa.org>

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