Add basic UTF-8 helper functions

Add utf8_width() and utf8_strwidth() which can be used to calculate the
display width of a single character or a string, respectively. A lookup
table is used to spot double width characters, as well as composing
characters. There currently isn't any code to deal with ambigious
characters.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -832,6 +832,10 @@ void          todo_view_note (char *);
 void          todo_init_list (void);
 void          todo_free_list (void);
 
+/* utf8.c */
+int           utf8_width (char *);
+int           utf8_strwidth (char *);
+
 /* utils.c */
 void         exit_calcurse (int) __attribute__((__noreturn__));
 void         free_user_data (void);