Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows

• projects linking to KCoreAddons to hide deprecated API up to a given version or silence deprecation warnings before a given version, using
  • -DKCOREADDONS_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKCOREADDONS_DEPRECATED_WARNINGS_SINCE
  • -DKCOREADDONS_NO_DEPRECATED_WARNINGS
  • -DKCOREADDONS_NO_DEPRECATED
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS
  • -DKF_NO_DEPRECATED
• to build KCoreAddons 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, 5.0.0, CURRENT and
all the versions where deprecations were done.

Reviewers: Frameworks, mpyne

Reviewed By: mpyne

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23800