Reorganize Property hierarchy

The super class of all concrete properties should be as abstract as possible to allow nearly-stateless implementations later.

This means for example:
DoubleProperty will be a new abstract class with no fields at all.
The current class DoubleProperty will be renamed to SimpleDoubleProperty.

As a side effect, all Expressions (e.g. DoubleExpression) do not extend from ObservableValueBase. Instead we will provide a class that allows to delegate the observable functionality to it and attach that to SimpleDoubleProperty, DoublePropertyBase, and DoubleBinding.