Kodi 17.4 and XBian upgrade process

Kodi have updated to version 17.4 which also brings some changes to XBian.

Upgrade process works as follows:

1. xbian-package-repo supplies a helper script (/usr/local/sbin/xbian-update-repo) which modifies sources.list and xbian.list. This script is only installed if system is still running under Debian Jessie
2. xbian-config GUI checks if this script is present and if not already running Stretch, it opens a message box to indicate a Distribution upgrade is available
3.  In that case, xbian-config GUI has a new function in category Update, which starts Distribution upgrade
4. When you start this operation, above script will be called, and after that a apt-get dist-upgrade will be executed.
   It should be noted that this operation usually takes a long time, and a system backup before running this command is recommended
5. After a successfully upgrade, this helper script is not longer needed and will be removed