ConstraintEditor UI polishing.
FIXED: ContraintEditor ratio constraint.
Merge branch 'Autolayout' of github.com:cacaodev/cappuccino into Autolayout
Try to fix required constraints

After this commit, CPLayoutPriorityRequired produces required
constraints in cassowary.
The windowView is edited with 510 weight instead of 1000 (says still
500).
InvalidateIntrinsicSizeTest passes.
Beginning of required failure handling.

Now, the engine outputs a warning indicating what required constraint
could not be satisfied. (Test this with ConstraintEditor).
Handle solve errors at the cappuccino API level.

Currently we just CPLog.warn if a solver error occurs.
TODO: hard errors in all cases except requiredFailure where we should
just warn , lower the priority, and replace the constraint.
CPWindow -layout optimization

Do not solve if no constraints were added/removed after a constraint
update.
FIXED: Finally got these editVar strength right.

Works in manual test, the why is still a bit mysterious.
To summarize what happens with strength:
User priority p 1-999 -> (strength, weight) = medium, p
Stay window = medium, 500
Edit window = strong, 1
CPLayoutRequired (including autoresizing constraints) = required, 1000.
On RequiredFailure, output the capp constraint.

Previously it was the cassowary constraint.
SolverError wraps casso Error (type, reason) and userInfo containing in
this case a ref to the capp constraint (-hash).