Add d-pointer to KReportSectionData and clean up the API and export it

Summary: The API needs to be public because e.g. public API of KReportDocument uses it.

Test Plan: Try kreportexample and KEXI reports

Reviewers: piggz

Reviewed By: piggz

Subscribers: Kexi-Devel-list

Tags: KReport

Differential Revision: https://phabricator.kde.org/D9893