Moved GNCDateEdit to completely use GObject system

[Stephen Evanchik]

Branch: gnucash-gnome2-dev

Moved GNCDateEdit to completely use GObject system:

        * src/gnome-utils/gnc-date-edit.h:
          Removed #include <gnome.h>
          Added #include <glib.h>
          Added private member 'disposed' to GNCDateEdit struct
        * src/gnome-utils/gnc-date-edit.c:
          Converted the following functions to use GObject:
                gnc_date_edit_get_type,
                gnc_date_edit_class_init
          Renamed gnc_date_edit_destroy to gnc_date_edit_finalize
          Added gnc_date_edit_dispose function that handles 
          child widget destruction
          Explicit casts to GTK_WIDGET in create_children