License Qt Wayland Compositor under GPLv3/Commercial

As announced to the mailinglist on October 10th,

Open-source licensing of Qt Wayland Compositor, Qt Application Manager and Qt PDF is to be changed from LGPLv3/Commercial to GPLv3/Commercial. Change becomes in effect with Qt 5.14 release. Going forward, these modules are no longer available under LGPLv3 license option.

This requires (at least) a change of the license headers in the source files, and other license-relevant files in the repository.

Since we have other mixed-license repositories (qtwebengine), having content within a single repository but under different licenses (wayland client code stays under LGPLv3) needs to be supported on the packaging side, which is being worked on in COIN-349.