Added mate-file-manager-gksu and dependencies

Signed-off-by: Willy Sudiarto Raharjo <willysr@gmail.com>
Update README for libgksu
Remove unneeded code in gksu
Fix slack-desc in mate-file-manager-gksu

Signed-off-by: Willy Sudiarto Raharjo <willysr@gmail.com>
Update KNOWN ISSUES
Add reference to upstream issues

Signed-off-by: Willy Sudiarto Raharjo <willysr@gmail.com>