Fix losing user files due to power outage short after saving in Krita

After we safe a file, we should ask OS to sync buffers to the hard-drive,
otherwise, the file will still be kept in memory. Therefore, if the
user loses power, the file will be lost.

The patch can only be implemented on Qt-level:
https://codereview.qt-project.org/c/qt/qtbase/+/276158