Respect plain text display preferences for all plain text content

Attachments with filenames are never parsed with Text::Quoted, so the
passed in $Message content was never an array reference.  This caused
textual attachments to ignore the PlainTextPre and PlainTextMono
settings, the symptom of which is nigh unreadable text.  Moving those
preferences out of the conditional corrects the display.  Now every call
to ShowMessageStanza with a text/plain content type will result in a
<div class="message-stanza">, which may be white-space preserving and/or
wrapped in a <pre>.
Merge branch '4.0/plain-text-attachment-display' into 4.0-trunk