gdkcontentserializer: Use newlines for file separators

Use newlines rather than spaces to separate file paths (or uri's)
when serializing text/plain files.  There isn't a matching
deserializer, so we can do this in isolation.  Newlines
seem to make more sense when pasting into a text editor etc.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5240

gdk/gdkcontentserializer.c

@@ -879,7 +879,7 @@ file_text_serializer (GdkContentSerializer *serializer)
           g_string_append (str, path);
           g_free (path);
           if (l->next)
-            g_string_append (str, " ");
+            g_string_append (str, "\n");
         }
       path = g_string_free (str, FALSE);
     }