Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows

• projects linking to KItemModels to hide deprecated API up to a given version or silence deprecation warnings after a given version, using
  • -DKITEMMODELS_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKITEMMODELS_NO_DEPRECATED
  • -DKITEMMODELS_DEPRECATED_WARNINGS_SINCE
  • -DKITEMMODELS_NO_DEPRECATED_WARNINGS
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_NO_DEPRECATED
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS
• to build KItemModels optionally with deprecated API excluded from the build, using "EXCLUDE_DEPRECATED_BEFORE_AND_AT" cmake argument.

Test Plan:
Builds with EXCLUDE_DEPRECATED_BEFORE_AND_AT set to 0, 4.8.0,
CURRENT.

Reviewers: Frameworks, dfaure, mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24600