…t was never necessary, and Unity throws exceptions for GUI control flow.

- Fixed nested arrays and lists accumulating too much indentation.
- Fixed label width to always truncate correctly. No more magic number.
- Truncate a copy of the the original label object to not affect the original.
- Made the GetScriptableObjectReference method protected so it can be used from derived drawers for deciding whether to render the base drawer when only a derived type should show properties.

You don't need to override GetPropertyHeight or OnGUI in derived property drawers, so I'm removing that from the README until version 1.1 is released.