Factor out UTF-8 code point decoding

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -1099,6 +1099,7 @@ int ui_todo_get_view(void);
 void ui_todo_set_view(int);
 
 /* utf8.c */
+int utf8_ord(const char *);
 int utf8_width(char *);
 int utf8_strwidth(char *);
 int utf8_chop(char *, int);