Explicitely set container on window view constraints because they are not added via addConstraint
CPLayoutConstraint : make mutable (again!) and allow subclassing via class constructor

TODO: error if user tries to mutate a constraint that is already in the
solver.
New: CPAutoresizingMaskLayoutConstraint class

Move mask -> constraints conversion to this class.
Tested in AppKit/CPLayoutConstraintTest.j

CPView: add stubs for NIMP methods.