Compatibility with Swiftmailer and different Mailing modules

Unfortunately I don't have a very deep knowledge of the Views Send module, so maybe it has to be the way it currently is for any reasons, but I don't think it's a good idea to rely on Mimemail so strong.

Recently I had to send emails containing inline images within via SMTP with Views Send and I didn't found any solution to get it working.

I tried the Mimemail module, the HTML Mail module in combination with Mail Mime, the SMTP Authentication Support module and combinations of them.
It wasn't possible at all to correctly send some mails with inline images.

Now I installed the Swiftmailer module and was able to get it working, but I still had to hack Views Send (see patch in the first comment) and had to add "image:" to each src attribute of the img tags (not the fault of Views Send; see Section 3.2.2).

Additionally the Mimemail module must stay enabled, although it isn't actually used, because otherwise Views Send would simply "destroy" my HTML.

That's just an example, but I think it's worth adding better support for other mailing modules and drop the heavy binding to Mimemail.