BT: multiple bugfixes with import/export of properties in a separate files

The branch is a good state for me and it can be merged. But there's still further work to do:

• Reexport all BT to rename files with better extensions.
• Using only the class name is not reliable enough. We need something better.
• For File/Image, I'd like to get rid off the _EtagSupport__etag & size properties. File._setData contains exactly the need code to restore the object. It would also solve the issue that big strings are currently not reimported as Pdata (OFS.Image).

/cc @georgios.dagkakis @kazuhiko