Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.

In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.

1 ) When first opened, the (user selected) post process shader config widget would print the wrong values on the text label next to int range sliders. For example if the range was from 1 to 6, and the value loaded from the config was 1, the label would print 0 when first opened, to then start showing the correct value once the slider was first moved.

This mirrors the behaviour of the float slider code below:
```auto* const value_box = new QLineEdit(QString::asprintf("%f", m_config_option->m_float_values[i]));```

2 ) The defautl int slider value would also be set wrong on first load, as it was being divided by the slider max instead of the slider step amount (again, just like for the float implementation). This is a mistake I had made with my previous submission.

Same fix as https://github.com/dolphin-emu/dolphin/pull/11846 but applied to int sliders instead of float ones

-The float sliders initial value wasn't calculated correctly
-Fix the checkbox dependencies not being applied until a setting was changed for the first time

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.