Add semantic data extraction plugin

Summary:
This plugins looks for structured data about the email content inside HTML
mail parts based on the schema.org ontology, as can be found in e.g.
airline and hotel booking confirmation emails.

The only thing this does with that information for now is showing a simple
summary of a flight itinerary. That's already useful, so you don't have
to look through the usual several pages of poorly rendered HTML content.
Allowing to add booking details to your calendar is an obvious next step.

Seeing how many workarounds were needed to parse the real-world mails I
have here, I suspect this will need more adjustments, so please send me
test material :)

See also https://developers.google.com/gmail/markup/

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8583