Summary:
Add license files are required by REUSE and add license
files as reuqired by REUSE.

Please note the following points:
- all links to dead webpages were removed from license statements
- I do not think that the ID markers are still needed, which are contained
  in several license headers (@mirko: are they still needed?)
- In both src/examples/ItemdDelegate.cpp/.h there is a license clarification
  that this file may be linked with any version of Qt. I do not see that
  this license clarification is needed, because that automatically comes
  with the LGPL. (@all: what is your opinion?)

Reviewers: cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, kde-frameworks-devel, mirko

Tags: #frameworks

Maniphest Tasks: T11550

Differential Revision: https://phabricator.kde.org/D27429