darwinkit: fix form,layout examples: "Content hugging priorities must be positive."

[progrium]

Seems calling appkit.View#SetContentHuggingPriorityForOrientation with any value (regardless of appkit.LayoutPriority const), causes this exception. Whether value is positive or not. Int or float. Not sure what's going on here.

Maybe the next step is to write some code in Objective-C (using cgo) to call this and see if it still happens.

Until then, this is preventing form and layout examples to work and could be an issue elsewhere in bindings.