Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows

• projects linking to Plasma to hide deprecated API up to a given version or silence deprecation warnings after a given version, using
  • -DPLASMA_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DPLASMA_NO_DEPRECATED
  • -DPLASMA_DEPRECATED_WARNINGS_SINCE
  • -DPLASMA_NO_DEPRECATED_WARNINGS
  • -DPLASMAQUICK_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DPLASMAQUICK_NO_DEPRECATED
  • -DPLASMAQUICK_DEPRECATED_WARNINGS_SINCE
  • -DPLASMAQUICK_NO_DEPRECATED_WARNINGS
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_NO_DEPRECATED
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS

Reviewers: Plasma, mart, davidedmundson, apol, mlaurent

Reviewed By: Plasma, mart, mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24678