(handle_fontified_prop): Do nothing if memory full.
(format_mode_line_unwind_data): New arg SAVE_PROPTRANS
controls whether to save and restore mode_line_proptrans_alist.
Callers changed.
(unwind_format_mode_line): Work with that feature.
(redisplay_internal): Don't call prepare_menu_bars if memory full.
(move_elt_to_front): New function.
(display_mode_element): Use move_elt_to_front.
Don't bother munging text props on a null string.
Delete obsolete elts from mode_line_proptrans_alist.
(decode_mode_spec): Test Vmemory_full, not spare_memory.