Clean up gwt-generated objects
gwt-modules: remove volatile nodes from manifests

The manifests are updated upon 'dist': file entries under
.../system/modules/.../resources/${module.gwt.bootstrap} are synched to
the corresponding nodes in the manifest. This commit removes all these
nodes as a first step towards a separation between source manifest and generated
manifest.
Rename ensuredirmanifest ant task to syncmanifest
Keep gwt-modules manifests clean

During the build, the manifest files of modules that use gwt are synched
against their corresponding gwt-autogenerated files. Furthermnore, the
parameter build-id is also modified (set to current date).

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.
Document param "gwt.buildid" in manifest.xml

The parameter is generated during the build (in concrete, during the
"dist" target).
.gitignore: ignore generated files (gwt-modules)
Merge pull request #153 from gallardo/patch-2c

Clean up manifest of modules using gwt