Improve render plugin selection

Summary:
This now considers two additional attributes:
- the message mimetype, with the most specific matching one considered
  first
- a plugin priority to disambiguate as a last resort

This became necessary with the fairly aggressive syntax highlighting
plugin also triggering on vcard or ical attachments. Their plugins now
have priority, as their mimetypes are more specific than text/plain the
highlighting plugin is registered for.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8195