RFC: Use SPDX license identifiers

According to KDE's licensing policies, a source file can have either SPDX markers or an old school license header. But the biggest annoyance with license headers is that it's very easy to overlook inappropriate licenses for kwin, e.g. GPL-3.0, plus we copy quite a lot of text every time a new file is added.

With SPDX markers, we just need to add a couple of special keywords and that's pretty much it. \o/

In order to create this change, I used licensedigger tool developed by Andreas Cord-Landwehr and a Python script to address some styling issues afterwards.