Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows

• projects linking to KTextWidgets to hide deprecated API up to a given version or silence deprecation warnings after a given version, using
  • -DKTEXTWIDGETS_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKTEXTWIDGETS_NO_DEPRECATED
  • -DKTEXTWIDGETS_DEPRECATED_WARNINGS_SINCE
  • -DKTEXTWIDGETS_NO_DEPRECATED_WARNINGS
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_NO_DEPRECATED
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS
• to build KTextWidgets 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.

Reviewers: Frameworks, mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24595