When exporting JAR description files from the JarPackageWizard, the
elements to be exported into the JAR file are currently stored in a
non-deterministic order. This makes it particularly difficult to merge
the description files in case they are put under version control.

This change makes a JAR description file exported from the
JarPackageWizard always list the exported elements in the same order by
sorting them according to their names. It also adds an according
regression test.

Fixes https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/1024