placesview: fix open action for locations without a mount or volume

In the "Other Locations" view, locations can be opened from the context menu
based on their mount or volume. However, some locations, like "Computer", do not
have either of those so they cannot be opened from the context menu. In order to
fix this, the file associated with the location can be used as well.

https://bugzilla.gnome.org/show_bug.cgi?id=768657

gtk/gtkplacesview.c

@@ -1468,11 +1468,20 @@ get_view_and_file (GtkPlacesViewRow  *row,
       mount = gtk_places_view_row_get_mount (row);
 
       if (mount)
+        {
           *file = g_mount_get_default_location (mount);
+        }
       else if (volume)
+        {
           *file = g_volume_get_activation_root (volume);
+        }
       else
-        *file = NULL;
+        {
+          *file = gtk_places_view_row_get_file (row);
+          if (*file) {
+            g_object_ref (*file);
+          }
+        }
     }
 }