Shadowed text field cell

Everyone loves shadows on text these days. It's hot shit but AppKit kinda sucks at it big time.

This wraps a simple API for setting shadow on a text field which can change depending on the cell's background style.

Classic use case here is labels within a cell view within a table view. Commonly you want a flat look when the cells aren't selected but something with a bit more pop when it is.

With this API you would simply set the shadow when the background style was `NSBackgroundStyleDark`.

You can also set a shadow which will be used as a fallback for all background styles (although explicitly set shadows will be preferred).