warn instead of error if removeConstraint fails. Small speed optimization in layout callback (about 2% faster)
Support for baseline attribute. Constraint Alignment rect instead of frames (cocoa).

New -baselineOffsetFromBottom CPView method.
Controls need to have an « alignment-rect-inset » theme value if we
want to align them instead of frames.
Currently has no effect. inset defaults to CGRectInsetMakeZero().
Visual test for baseline attribute in WorkerTest.
Use the new designated initializers for Expressions.

Replace integers in Expression / Constraint initializers with Variable
and Expression respectively.
Use variable, not constants, in sizeconstraints : width >= [value:k] NOT width >= k
When the intrinsicContentSize changes, update this constant by suggesting a new value.
This is simpler and cleaner than removing/adding the constraints. Fixes a bug in CPLayoutConstraintCib test when changing twice the intrinsicContentSize of the middle button.
REGRESSION: in intrinsicContentSize test : hugging button jumps to an uncompressed state when we change the size constraint although in this exemple anti-compression pririty is very low
(we change cassowary constraints for both hugging and anti-compression).