Uninstall-Option: leave userdata

[HaJo]

The uninstall currently removes ufo:ai completely.

It would be nice to have an option "leave data",
e.g. don't delete savegames, screenshots, config-files, logfiles.

Submitted as issue 1532527 sf.net:
https://sourceforge.net/tracker/index.php?func=detail&aid=1532527&group_id=157793&atid=805245