Occlusion and constraint lookup fixes.

[FIXED]     UIEdgeInsetsForRectsubtractingRect when rects don't intersect.
[FIXED]     firstConstraintForAttribute: consider all the constraints applicable to the view.
[FIXED]     insetOcclusion: Don't inset horizontally if vertical inset is sufficient to remove occlusion.
[ADDED]     applicableConstraints: all constraints that affect a view.