Qtdemux refactor

This is some refactoring I did at last year's Oslo hackfest as preparation for adding 360 degree support (https://gitlab.freedesktop.org/thaytan/gst-plugins-good/commits/spatial-media-isomp4). I'm not quite ready to merge the 360 atom support yet, but getting the refactoring in would prevent conflicts in rebasing that branch and I think makes qtdemux a little tidier - which can only be good.