Time4J-ui based on JavaFX needed as an extra project

[MenoData]

The module time4j-ui which relies on JavaFX cannot any longer run in Java-environments starting with Java 11 because the FX-component was removed from standard Java. Hence it is appropriate

a) to remove the module time4j-ui from existing modular time4j-project and
b) to change time4j-ui to an extra project named time4j-fx for environments where users have installed JavaFX. This approach is comparable with the timezone extension project tzdata and can better deploy special maven build plugins.

Step a) will be done with version v5.8.1.