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>.