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/Makefile.am

@@ -22,6 +22,7 @@ calcurse_SOURCES = \
 	recur.c \
 	sigs.c \
 	todo.c \
+	utf8.c \
 	utils.c \
 	vars.c \
 	wins.c \