Split online help code into a separate function

Reintroduce help.c and move the online help code into a new function
display_help().

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

src/calcurse.h

@@ -723,6 +723,9 @@ void event_paste_item(struct event *, long);
 enum getstr getstring(WINDOW *, char *, int, int, int);
 int updatestring(WINDOW *, char **, int, int);
 
+/* help.c */
+int display_help(const char *);
+
 /* ical.c */
 void ical_import_data(FILE *, FILE *, unsigned *, unsigned *, unsigned *,
 		      unsigned *, unsigned *);