WIP: Arrangement order of C++ headers and sources

KDevelop always arranges headers left of sources with the "Arrange related documents side by side" setting. Somehow I don't want to get used to this and prefer sources left of headers instead, which always requires me to re-order documents after opening them. I would thus like to have a setting where the desired order can be set. However, I don't know whether anyone else cares about this and if yes, where the best place for that setting would be. So, for now I'll only put a WIP commit here which changes the arrangement order, and if anyone else should be interested, I could add a setting for it if a suitable place is found. One candidate is the "User Interface" page, but this is supposedly not language-specific. Another candidate is the C/C++ Helpers page, which is at least language-specific, but otherwise much less related to this specific setting.