Expose term_translate outside terminal.c.

Also, instead of insisting on modifying the UTF-8 decoding state
inside the Terminal structure, it now takes a separate pointer to a
small struct containing that decode state. The idea is that if a
separate module wants to decode characters the same way the real
terminal would, it can pass its own mutable state structure, but the
same main Terminal pointer.

defs.h

@@ -28,6 +28,7 @@
 
 typedef struct conf_tag Conf;
 typedef struct terminal_tag Terminal;
+typedef struct term_utf8_decode term_utf8_decode;
 
 typedef struct Filename Filename;
 typedef struct FontSpec FontSpec;