Various Config Improvements

We made some more aspects of the PDF configurable.
This, together with _modFooter makes it possible to omit details in the footer. I think we don't want this overhead in the engine. But I like the idea. So we should have an event which makes it possible to unset certain entries.

And I didn't find any config setting for `sales_pdf/invoice/footer_imprint`. Did you wrote it with a install script to the database?