Remove QSettings from QtCore

The implementation of QSettings is not up to the standards we would like to have. It is also questionable whether ini files are the best way to do a persistent storage. Thus QSettings should be moved to a add-on to Qt and Qt itself should not depend upon it.

• Remove dependencies to QSettings in QtCore and QtGui (QtWidgets can still depend upon it).
• Move QSettings out into a module of it's own.