KWayland: Convert license headers to SPDX

Summary:
Convert license headers to SPDX expressions and add license files as
required by REUSE specification.

Reviewers: zzag

Reviewed By: zzag

Subscribers: kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T11550

Differential Revision: https://phabricator.kde.org/D28058