New: KReportAddQCH.cmake for creating an API dox QCH file during the build

Summary:
Custom copy of upcoming ECMAddQCH.cmake, see https://phabricator.kde.org/D2854
for existing issues and open questions.

Test Plan: KReport.qch generated by this: https://share.kde.org/index.php/s/KmkOqGZV9jA6Jvf

Reviewers: piggz, KReport, staniek

Reviewed By: KReport, staniek

Tags: KReport

Differential Revision: https://phabricator.kde.org/D3515