Implement include-what-you-use policy

[BenBE]

This implements the include-what-you-use policy to clean up cross-references between headers and clearly show what actually is used inside a file. While this is by far not the minimal set of headers allowed to be included, this gives a good and stable set of includes to avoid breakage if something is moved around. Also the tool iwyu allows to detect headers that are included but never used.

This PR includes three patches:

1. Cleanup of includes in all source and header files
2. Basic implementation to use IWYU from the Makefile
3. Some fine-tuning to disable IWYU by default as its output can be quite verbose at times

Note that IWYU uses a fixed clang version. Thus if IWYU is compiled to use a certain clang version you need to provide the proper clang binary and system include files or IWYU will report missing files even though normal compilation succeeds. See the documentation of Include-What-You-Use for details.