Clean up manifest of modules using gwt

During the build, the manifest files of modules that use gwt are synched
against their corresponding gwt-autogenerated files. Furthermore, the
parameter build-id is also modified (set to current date). As these files are under version control, a build results in a lot of files being marked as modified.

This commit modifies the old ensuredirmanifest task (now syncmanifest),
so it does not overwrite the source manifest anymore. The synched
manifests are saved now into temporary files and packaged as
manifest.xml during the "dist" phase. The temporary files are deleted
afterwards. This way, the original manifest.xml remains unmodified and
doesn't trigger any source control inconsistency.

Also see http://lists.opencms.org/pipermail/opencms-dev/2013q1/038002.html