New: KPropertyAddQCH.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:
KPropertyCore.qch generated by this: https://share.kde.org/index.php/s/y8s0oAkxUNc1dXl
KPropertyWidgets.qch generated by this: https://share.kde.org/index.php/s/anNMRKLBGDxTSPe

Reviewers: piggz, KProperty, staniek

Reviewed By: KProperty, staniek

Tags: KProperty

Differential Revision: https://phabricator.kde.org/D3514