Support for CPContentSizeLayoutConstraint -setConstant: (not used). Updated exemples to -layout window from AppController (TODO: layout shoudl be done by the framework)
 Fixed: cassowary wrapper was global in sync mode.

Also moved callbacks from engine to window in an attempt to make the
layout engine separate from the ui system. TODO: engine delegate or a
more generic callback & a constraint item protocol.
Added constants for some internal priorities.
Cocoa/capp comparison example : added size to fit with popup.
Autolayout is now 10% slower than autosize. TODO: find why.
Add cibs announced in previous commit