Remove useless includes (using include-what-you-use)

[JohannesLorenz]

Enhancement Summary

There's a tool to find them:

cmake -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-Xiwyu;--mapping_file=/usr/share/include-what-you-use/qt5_11.imp" -DCMAKE_C_INCLUDE_WHAT_YOU_USE="include-what-you-use;-Xiwyu;--mapping_file=/usr/share/include-what-you-use/qt5_11.imp" ..

If the tool is named "iwyu" instead, use that in the CMake command above.

The tool gives headers that should be

• added if you follow the principle to include every header if you use a definition from it (decided against that in discord)
• removed (because they are useless)

The tool gets a bit confused by Qt, it asks to replace stuff like by "qstring.h" - this can be ignored.

When you submit a PR it should not contain any removement suggestions.

Justification

Compile time matters, and I wonder how much we would win if we would remove all useless includes.

[Spekular]

This should be closed, right?

[JohannesLorenz]

Yes. Thanks for the reminder!