Open focused file with whatever is appropriate

While in the "Files" view of a torrent's Details, hitting enter on a
focused file will execute the following command, in a nonblocking
manner:

    xdg-open <focused file's path>

This will, of course, not work if the client is not launched from the
same machine as the server, because it isn't the same file structure.
If, that is the case, the operation will fail silently. On the other
hand, if the user has the current torrent files on the "client" machine
with the same path as the server, he may be amazed as how
"transmission-remote-cli" figured out where his file was. :)
Fix wrong file index when tryign to open a file

The index used belonged to the row in the client's array of files instead of
the server's.
Catch OSError if xdg-open fails (maybe not installed)