Summary:
Allows
* projects linking to KXmlGui to hide deprecated API up to a
  given version or silence deprecation warnings after a given version,
  using
  * -DKXMLGUI_DISABLE_DEPRECATED_BEFORE_AND_AT
  * -DKXMLGUI_NO_DEPRECATED
  * -DKXMLGUI_DEPRECATED_WARNINGS_SINCE
  * -DKXMLGUI_NO_DEPRECATED_WARNINGS

  or
  * -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  * -DKF_NO_DEPRECATED
  * -DKF_DEPRECATED_WARNINGS_SINCE
  * -DKF_NO_DEPRECATED_WARNINGS
* to build KXmlGui 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, 4.1.0, 5.0.0,
CURRENT.

Reviewers: #frameworks, dfaure, mlaurent

Reviewed By: dfaure, mlaurent

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D24466