Follow-Up of 7db3fa94a1 .
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.
This PR places all LMMS symbols into namespaces to eliminate any potential future name collisions between LMMS and third-party modules.
Also, this PR changes back `LmmsCore` to `Engine`, reverting c519921306 .
Co-authored-by: allejok96 <allejok96@gmail.com>
