(Qdisable_composition): New variable; delete `Qcomposite'.

(Fmake_coding_system): Add new property `disable-composite'; delete
property `composite'.
(COMPOSE_ADD_CHAR): Use `CODING_SYSTEM_DISABLE_COMPOSITION' instead of
`!CODING_SYSTEM_COMPOSITE'.
(syms_of_file_coding): Add new symbol `disable-composition'; delete
symbol `composite'.