New: KDbAddQCH.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: KDb.qch generated by this: https://share.kde.org/index.php/s/wpRRWfYeZKaccjN

Reviewers: piggz, staniek, KDb

Reviewed By: staniek, KDb

Tags: KDb

Differential Revision: https://phabricator.kde.org/D3513