Provide persistency of installed apps

As a user I would like to keep installed apps after a power off of the system on target or a shutdown of the Neptune UI on the desktop. As of 5.12 all apps installed during the time when Neptune is up, are deleted after a shutdown.

If we would use standard Qt paths, it is currently not possible to specify such a value in the "installationPath" for appman, since it expects real file system paths.