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.
* Remove QTextCodec
QTextCodec was removed from Qt6 and is only available through the
Qt5Compat module.
QTextCodec was only used by the HydrogenImport plugin when importing old
Hydrogen files that were saved using TinyXML before it supported UTF-8.
HydrogenImport would use QTextCodec to try to get the current encoding
from the locale, and then use that as a best guess for interpreting the
XML data in the unspecified encoding it was saved in. None of this was
ever reliable, since the encoding of the computer that saved the
Hydrogen file might not be the same as the computer running LMMS and
importing that file.
There is no good solution here, so I decided to simply assume the old
Hydrogen files are UTF-8 encoded. The worst that can happen is someone's
ancient Hydrogen files containing non-ASCII text of some random encoding
becomes mojibake'd after importing into LMMS, which is something that
already could have happened.
* Clean up a little
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>
