Release xft_data in widget destroy callback to avoid visual distraction

Release xft_data in widget destroy callback to avoid visual distraction

* lwlib/lwlib-int.h (struct _widget_instance) [HAVE_XFT]: Remove nr_xft_data.
* lwlib/lwlib-Xaw.c (find_xft_data, xaw_update_one_widget) [HAVE_XFT]: Loop
while widget member is not NULL instead of using nr_xft_data.
(xaw_destroy_instance) [HAVE_XFT]: Move xft_data release code from here ...
(destroy_xft_data) [HAVE_XFT]: ... to here.
(make_dialog) [HAVE_XFT]: Add destroy_xft_data as destroy callback for dialog.