(mode_line_string_list, mode_line_string_face)

(mode_line_string_list, mode_line_string_face)
(mode_line_string_face_prop): New variables.
(store_mode_line_string): New function.
(display_mode_element): Use store_mode_line_string to
add mode-line string elements to mode_line_string_list
when mode_line_string_list is non-nil.
(Fformat_mode_line): Now returns propertized string by
default.  New arg NO-PROPS to ignore properties.
(decode_mode_spec): Only add two dashes for %- in propertized
mode-line string.
(syms_of_xdisp): Init and staticpro mode_line_string_list.