When the prefer-plain is set to show only text/plain parts, without showing
suppressed HTML parts as attachments, and the received message contains
only one part, the text/html part, then the preview was completely empty,
which is confusing. With this change, the text/html only messages will
show the suppressed part as attachment regardless the setting.
Fix regression from commit 99a875ed which has broken displaying
of suppressed HTML parts as attachments when the HTML part is embedded
in a multipart/* container.
audio-inline, itip-formatter, prefer-plain, tnef-attachments
and vcard-inline plugins were converted to modules so that they
can fit into concept of the new formatter.
Every module still installs .eplug file, because there is no
suitable API at the moment to register plugins to the plugins dialog
and to extend the Preferences dialog.
