Higher level subclassing API

[progrium]

This PR introduces objc.NewClass[T](), which takes a struct type implementing IObject that it uses to allocate a class. It uses reflection to set the class name from the struct type name, and uses NSObject as the super unless specified by a struct tag. It optionally takes selectors that will be added as methods using the struct methods with Go equivalent name. The resulting class can be registered and has a type specific New() that initializes and autoreleases.

Example

type CustomView struct {
	appkit.View `objc:"NSView"`
}

func (v CustomView) AcceptsFirstResponder() bool {
	return true
}

func (v CustomView) KeyDown(event appkit.Event) {
	log.Println("Keydown:", v.Class().Name(), event.Class().Name())
}

func main() {
	// passing a selector that doesn't have a matching Go method will panic
	CustomViewClass := objc.NewClass[CustomView](
		objc.Sel("acceptsFirstResponder"),
		objc.Sel("keyDown:"),
	)
	objc.RegisterClass(CustomViewClass)

	// New() returns a CustomView, but InitWithFrame() will return a View.
	// If you need CustomView, you can assign it before the View init method.
	view := CustomViewClass.New().InitWithFrame(rectOf(0, 0, 150, 99))

	...
}

More examples can be found in the tests or the updated subclass example.

Breaking Changes

Changes the Class() method on objc.Class and objc.IClass to MetaClass().

[tmc]

Nice, I ilke the interface.