open sla files with Scribus

*.sla files should be opened with Scribus

Confirmed on Tails 0.12.1 and 0.14~rc1. The "choose application" dialog that pops-up has ScribusNG in the list, though, so this is not as bad as it seems.

This should be fixed for free in Wheezy (#6015 (closed)), as scribus-ng there is only a transitional package that depends on scribus, so let’s wait.

Original created by @tails on 5642 (Redmine)