Add ability to set up native control when subclassing NativeControlHost

[cwensley]

This gives a better pattern for subclassing the NativeControlHost and creating the native control using a callback.

A few tweaks to the naming (controlObject > nativeControl), and ensure the ToEto() helpers for each platform uses the NativeControlHost instead of hooking up manually.

E.g.

class MyControl : NativeControlHost
{
	protected override void OnCreateNativeControl(CreateNativeControlArgs e)
	{
		base.OnCreateNativeControl(e);
		e.NativeControl = yourNativeControlOrHandle;
	}
}