solid_boolean() collects the selected solids straight out of
scene.selected, a HashSet, and then uses handles[0] and handles[1] as the
two operands. Set iteration order is arbitrary and can differ between
runs, so SUBTRACT — the one non-commutative operation of the three — could
cut either solid out of the other for the same selection.
Sort the pair by handle, so the solid created first is the body the other
is taken out of. UNION and INTERSECT are unaffected.