Selecting the file when opening folder of a single file torrent

[migge]

The attached patch makes the GTK+ client select the file of a single file torrent when using "Open Folder" from the context menu on it. This saves the user from having to search for the file manually, which is useful if the folder contains many other files. For now only Nautilus is supported, but for example Nemo support could be added with a single line of code. If the user does not use Nautilus as the default file manager the old behaviour (not selecting the file) is used as a fall-back.