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.
Are there other possible values for ``modification`` planned? If not, I'd suggest making it a boolean instead of a string in order to improve the validability (which might not even be a word), e.g., ``<RequiredBundle inherit_modification="true" name="foo"/>``. We might be able to come up with a simpler/shorter name than ``inherit_modification``. Of course, if there's a good reason to keep it a string I'm all for it.