Three issues with the existing SimpleStyleable<T>Property classes

1. The classes are marked as abstract, and yet there are no abstract methods that need to be overridden.

2. The JavaDoc claims that getCssMetaData needs to be overridden, but it is already implemented and marked final.

3. By extending from Simple<T>Property instead of Styleable<T>Property the utility of Styleable<T> is weakened, since there is no pre-supplied trivial implementation of a Styleable<T>Property.