Remove lots of useless/misplaced includes

[JohannesLorenz]

Follow-Up of 7db3fa9 .

This was done by setting CMAKE_C_INCLUDE_WHAT_YOU_USE and CMAKE_CXX_INCLUDE_WHAT_YOU_USE to (broken down into multiple lines here, note, all below FL/x.h is not required for C):

include-what-you-use;
    -Xiwyu;--mapping_file=/usr/share/include-what-you-use/qt5_11.imp;
    -Xiwyu;--keep=*/xmmintrin.h;
    -Xiwyu;--keep=*/lmmsconfig.h;
    -Xiwyu;--keep=*/weak_libjack.h;
    -Xiwyu;--keep=*/sys/*;
    -Xiwyu;--keep=*/debug.h;
    -Xiwyu;--keep=*/SDL/*;
    -Xiwyu;--keep=*/alsa/*;
    -Xiwyu;--keep=*/FL/x.h;
    -Xiwyu;--keep=*/MidiApple.h;
    -Xiwyu;--keep=*/MidiWinMM.h;
    -Xiwyu;--keep=*/AudioSoundIo.h;
    -Xiwyu;--keep=*/OpulenZ/adplug/*;
    -Xiwyu;--keep=QPainterPath;
    -Xiwyu;--keep=QtTest

FAQ:

• Q: Does this speed-up a completely fresh compile?

• A: No, I measured it.

• Q: Does it speed up anything else?

• A: Yes. If you change one header, it can reduce the number of CPP files that your compiler needs to recompile, or your IDE has to re-scan.

• Q: What other reasons are for this PR?

• A: It's idiomatic to only include headers if you need them. Also, it will reduce output for those who want to use IWYU for new PRs.

Background:

This is just a remainder PR of what I planned. My original idea was to setup a CI which warns you of useless includes (but not of all issues that IWYU complains about). However, I could not see that this was favored on Discord. A full IWYU CI also has the problem that it (possibly??) needs to compile with make -j 1, which would make CI really slow.

However, for that plan, I had to fix the whole code base to be IWYU compliant - which it now is.

[JohannesLorenz]

For reference: For zyn we have planned an action which greps for these unnecessary includes, and here is example output of the job.

[allejok96]

Nice housekeeping. I just glanced through it. If it builds it works™

[allejok96]

What does this have to do with IWYU? just curious, not even trying

[JohannesLorenz]

LmmsCommonMacros.h only defines two macros: LMMS_STR and LMMS_STRINGIFY. Both are used in embed.h only if PLUGIN_NAME is defined. So the change seems sane.

My guess why I did this: I probably compiled a CPP file which include embed.h while PLUGIN_NAME was undefined - so IWYU likely told me: "You're including that header, but in this translation unit, you are not using anything from that header" - so I wrapped the #if around the include to make IWYU happy.

[JohannesLorenz]

This PR still need one approval of a reviewer with write access. I guess this should be easy to finish, after 2 devs have already reviewed?

[messmerd]

LGTM