SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
A C++ source file containing just
#include <LibFoo/Bar.h>
should always compile cleanly.
This patch adds missing header inclusions that could have caused weird error
messages if they were used in a different context. Also, this confused QtCreator.
