Added rcube_message::has_text_part(), simplified has_html_part() so it always
works in "recursive mode" - removed $recursive argument.
Enriched content should be also converted to text if html is disabled
Use $mime_parts not $parts in has_*_part() methods so detection is
correct no matter if prefer_html is enabled or not.
Fix has_*_part() methods so they return same result no matter what
prefer_html option value is
Add ability to toggle between view as HTML and text while viewing a message (#1486939)