Use ECMGenerateExportHeader to manage deprecated API better

Allows

• projects linking to KAuthCore to hide deprecated API up to a given version or silence deprecation warnings after a given version, using
  • -DKAUTHCORE_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKAUTHCORE_NO_DEPRECATED
  • -DKAUTHCORE_DEPRECATED_WARNINGS_SINCE
  • -DKAUTHCORE_NO_DEPRECATED_WARNINGS
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_NO_DEPRECATED
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS

• to build KAuthCore optionally with deprecated API excluded from the build, using "EXCLUDE_DEPRECATED_BEFORE_AND_AT" cmake argument.