Dynamic theming per newsletter for blocks and emails

The attached patch enables the blocks, newsletter body and footer to be themable per newsletter term.
With the patch the newsletter body can be themed using these files in your theme directory:
* for all newsletters: simplenews-newsletter-body.tpl.php
* per newsletter: simplenews-newsletter-body--[tid].tpl.php
([tid] to be replaced by the term's id)

Patch commited to HEAD and 6.x.
(issue created for reference only)