Fix all krazy #include warnings

- "include own header first line"
- "put config.h in angle brackets line"
- "do not include QtModule/QtClass line"

`QElapsedTimer` was implicitly included by
kfileitemmodelsortalgorithm.h, now we need to explicitly include it in
kfileitemmodel.cpp.

We also need to explicitly link to `Qt5::Concurrent`, otherwise we
cannot `#include <QtConcurrentRun>`.