Automatically unpack LWJGL in the Java backend.

This eliminates all the fiddly "make your IDE or build system understand that
we're using LWJGL and set things up for you" pain in the rear.

It also enables "mvn package" to trivially create a single standalone jar file
that will run the Java version of your game, which is now in the archetype.

This does require relegating interactive testing of the Java backend back to
"mvn test -Pjava" instead of just "mvn test", because Maven sucks and uses a
ridiculous inextensible "lifecycle" model instead of targets. Sigh.