Focus password field after selecting a label
Select label options after FocusOut event

- dispass/gui.py (GUI.labelSelected): Only change focus, let
  `labelFocusOut' handle the switching of options.

  (GUI.labelFocusOut): New function.
Filter labels in combobox by given value

Filter the possibly selectable labels for the label combobox by the
value in the entry.

Since the filter return all values when the input is empty there is no
need to explicitly set the `values' option for the combobox.