Use explicit values for constraint strength

Use explicit values for constraint strength

Instead of playing games with mapping negative symbolic values to
positive ones, let's use the appropriate constants everywhere. This
allows us to use:

        GTK_CONSTRAINT_STRENGTH_WEAK * 2

Or

        GTK_CONSTRAINT_STRENGTH_STRONG + 1

In code using the public API.

We also store the strength values as integers, so we can compare them
properly, and only turn them into doubles when they are inserted into
the solver, just like every other variable.