Clean up manifest of modules using gwt

(This is a "remake" of pull https://github.com/alkacon/opencms-core/pull/149 according to the comments up to https://github.com/alkacon/opencms-core/pull/149#issuecomment-14173112)

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 alert.

Also see http://lists.opencms.org/pipermail/opencms-dev/2013q1/038002.html