The main advantage of SPDX license identifiers over the traditional
license headers is that it's more difficult to overlook inappropriate
licenses for kwin, for example GPL 3. We also don't have to copy a
lot of boilerplate text.
In order to create this change, I ran licensedigger -r -c from the
toplevel source directory.
This test mocks part of KWin::Udev, udev and libinput. The test itself
is still rather limited and only verifies whether libinput is valid or
not and that assignSeat works. Most of the interaction is not yet tested,
though to a large degree doesn't make sense and should be rather tested
in the context of LibInput::Connection.
