Bundler: add modification support to Bundle dependencies

Bundle dependencies are now realized with RequiredBundle and support inheritance of the modification flag. This requires new client support and will only work with clients >= 1.4.0pre2.

The new tag name is required for validation. The tag name and type association in the schema has to be unambiguous and we need nested bundle tags for xincludes. Bundle dependency are only available in 1.4.0pre1, so I do not know if we need a migration script that renames the empty bundle tags to RequiredBundle.
No it is ``bundle not in self.modified`` because we do only want to add bundles to self.modified that are not already in there. The BundleUpdated hook gets called for every bundle (even if there is no tag inside that is handled with BundleDeps) and BundleDeps searches for all tags that are referencing the current bundle name from other bundles. So the ``bundle not in sef.modified`` should simply eliminate duplicate entries.