Add compatibility with Java 9 / Jigsaw

[Don-vip]

Java 9 is well advanced in its development, early builds are now available. ​Jigsaw project is not yet merged into main tree, but provides its own binaries and javadoc:

​http://openjdk.java.net/projects/jigsaw/ea
​http://download.java.net/jigsaw/docs/api/

I have added basic Java 9 support in r8777:8778, r8793, r8795, r8796:8797 but making JOSM work with Jigsaw is another challenge.

This ticket aims to list all problems encountered.

1. xmltask does not work ==> cannot build OSX package ==> ​fork created
2. ​Proguard does not work ==> cannot build optimized jar
3. ​Groovy does not work ==> cannot run some tests + cannot build taginfo files
4. ResourceBundle.getBundle("sun.awt.resources.awt") results in an internal error ==> bug ​reported to Oracle

[Don-vip]

In 8816/josm:

see #11924 - JDK9 Jigsaw: workaround for ​https://bugs.openjdk.java.net/browse/JI-9025152

[Don-vip]

In 8817/josm:

see #11924 - JDK9 Jigsaw: do not run code in headless environment (for jenkins unit tests)

[Don-vip]

In 8826/josm:

see #11924 - update xmltask to a new modern version built from ​https://github.com/don-vip/xmltask