Update layer properties when using Rebel animator proxies

[jspahrsummers]

When setting view properties on an AppKit animator proxy, those changes are not reflected on the view or the proxy until the animation completes. This is a huge defect, because it becomes extremely hard to write layout code that works whether an animation is in progress or not.

Rebel's animator proxies could solve this by writing the corresponding layer properties after using the AppKit proxy, since the layer will update immediately to reflect the change. Similarly, using a proxy to read properties could check the layer for in-progress changes.

An example of this sort of pattern exists in the GHfM diff view implementation.