Various improvements to Newsletter tab page

Committed: http://drupalcode.org/project/simplenews.git/commit/414e30e.

Thanks !

"node/%node/simplenews" page has to be admin?

If "Send a test newsletter to the test address" is selected and submit the form. Theme files as simplenews-newsletter-body.tpl.php which are the main theme never be called, being an admin page

It's possible the change of "Use the admin theme for the page (equal to the admin theme setting for the edit node page)" it's not necessary? .
Because when test mail is sended. The templates used on html mail are admin theme and not the default theme

I attach a patch that remove this change.

I'd rather fix the fact that wrong templates are used (see #1071174: Template files do not work when using admin theme (Drupal 7), for example) rather than rolling back this change.

Possibly options/settings are needed to meet all possible needs..
Note all those cases cover much complexity. So i'm kindly asking for wise decisions before adding too much change (like this issue already did).

The theme problem is being fixed in #374222: Template files do not work when using admin theme.

Other improvements to the newsletter tab are being discussed at #1169990: Improve usability and displayed information on the newsletter tab.