Add a generic list box implementation

This adds a very generic list box implementation. List boxes with items
of different heights are supported. Two callback functions to determine
the height of every single item and to draw a specific item are used.

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

src/Makefile.am

@@ -23,6 +23,7 @@ calcurse_SOURCES = \
 	ical.c \
 	io.c \
 	keys.c \
+	listbox.c \
 	llist.c \
 	note.c \
 	notify.c \