Extract document metadata into separate behavior

- Refactored `opengever.document.behaviors` into a package with separate modules
- Moved all document metadata fields except `title` and `file` from the `IDocumentSchema` to a new schema behavior `IDocumentMetadata`
- Updated document overview template according to new widget names
- Added form directives to retain field ordering in Document add / edit forms
- Updated default value adapters where necessary
- Updated registration for indexers to new `IDocumentMetadata` schema where necessary
- Updated event handlers to not rely on fields being present at event time - defaults for schema behaviors with `AttributeStorage` get set very late, and may not exist yet during events
- Re-implemented validator for "require file or preserved as paper"
- Updated affected tests

@phgross ready for review.
Wrong order ...