Use ReadOnlyBooleanProperty instead of ObservableValue<Boolean>

[daniel-shuy]

First off, thanks for this library.

This PR modifies DirtyProperty#isDirtyProperty(), DirtyBooleanProperty#originalValueProperty() to return ReadOnlyBooleanProperty instead of ObservableValue<Boolean>, and modifies CompositeDirtyProperty to extend ReadOnlyBooleanProperty instead of ObservableValue<Boolean>.

The reason for this is that ReadOnlyBooleanProperty has some useful utility methods, eg. BooleanExpression#not().