Fix plain text template

Summary:
Because of the missing "page_template.txt" include, if one exported a page, produced
txt files were empty.

Test Plan:

1. Open some page
2. Try `File -> Export -> To Text File..."

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29053