#2 Fix Gtk warning about unterminated column list

[Stefan Achatz]

On creating the filelist of the calculator gtk complains with
(hptalx:9264): Gtk-WARNING **: gtkliststore.c:797: Invalid column number 16 added to iter (remember to end your list of columns with a -1)
for each item.
A terminating -1 in a gtk_list_store_set() function call was added to fix this.

[Stefan Achatz]