Add RTF support to Okular

Summary:
Adds RTF support to Okular
BUG: 339835

Test Plan:
Okular correctly renders RTF documents.
RTF is added to Okular's mime filter

Reviewers: #calligra:_3.0, #okular

Subscribers: kossebau, aacid, ltoscano

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D7431