Also support multipart/alternative with text/plain and multipart/related as children, instead of the usual text/plain and text/html.
Also support multipart/alternative with text/plain and multipart/related as children, instead
of the usual text/plain and text/html.

This fixes that the content of some Newsletters with images was rendered twice, bot the HTML and
the plain text part was shown.

BUG: 187989

svn path=/trunk/KDE/kdepim/; revision=1002770

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi