Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows
* projects linking to KWayland to hide deprecated API up to a
  given version or silence deprecation warnings after a given version,
  using
  * -DKWAYLAND_DISABLE_DEPRECATED_BEFORE_AND_AT
  * -DKWAYLAND_NO_DEPRECATED
  * -DKWAYLAND_DEPRECATED_WARNINGS_SINCE
  * -DKWAYLAND_NO_DEPRECATED_WARNINGS

  or
  * -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  * -DKF_NO_DEPRECATED
  * -DKF_DEPRECATED_WARNINGS_SINCE
  * -DKF_NO_DEPRECATED_WARNINGS

No support for "EXCLUDE_DEPRECATED_BEFORE_AND_AT", needs to be done by
someone with detailed knowledge about disabling implementation, if wanted.

Reviewers: #kwin

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D24663