[conf.xml][GUI] GUI for selecting the desired active list of airframes.

First attempt to gather feedback / ideas.

- 100% compatible parallel addition: if you just want to keep your conf.xml as before, that is fine
- suggests to place own conf in a conf.xml.personal file, and to add it to your git repo
- personal conf can be based on any other conf, not just examples but also the more reliable tests_conf
- easy conf switching: e.g. if after some time your conf is broken and you want to quickly check the example conf again to see if that still works
- auto-backup of existing conf.xml (if not a symlink)
- auto-backup of existing conf.xml.personal

todo:

- [ ] do same for controlpanel.xml
- [ ] replace os.system calls with less system specific commands
- [ ] make GUI look nicer
- [ ] upon selecting some conf (e.g. examples), add a ImportAirframeToMyPersonalConf button showing a selection dialog with that confs airframes. Then xml-add the selected airframe to your conf.xml.personal.


accidently closed #534 due to bad branch name