Use a GtkComboBox in place of the deprecated GtkOptionMenu.

[David Hampton]

Log Message:
-----------
Use a GtkComboBox in place of the deprecated GtkOptionMenu.  Add a
choice in the combo box for the UTC time format to match the new
addition to the QofDateFormat enum.  Clean up the code slightly.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/gnome-utils:
        gnc-date-format.c
        gnc-date-format.glade
        gnc-date-format.h