specific Business Template parameter (template_registered_skin_selection).
This modification will help adding an existing skin folder to a new skin
selection.

Skin folder exported after this revision will not have the
business_template_registered_skin_selections properties anymore, and so, it
will be required to update the business template configuration.

Compatibility with existing bt5 is supported by allowing imported skin folder
with this property.

Thanks to Kazuhiko for the review.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@28759 20353a03-c40f-0410-a6d1-a30d3c3de9de