Add option to create QCH file with public API dox, using ECMAddQCH

Summary:
Allows e.g. packagers to generate QCH files during build, for automated
distribution of latest docs matching the installed library

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D24057