Drop empty notes after editing.

Keeping empty notes doesn't make sense here. Also, there doesn't seem to
be a simple way to erase notes yet. This will make calcurse delete any
notes that are empty (meaning that they are either 0-byte files or
contain nothing but a newline character) when returning from the editor.

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

src/calcurse.h

@@ -696,6 +696,7 @@ void             io_stop_psave_thread (void);
 void             io_set_lock (void);
 unsigned         io_dump_pid (char *);
 unsigned         io_get_pid (char *);
+int              io_file_is_empty (char *);
 
 /* keys.c */
 void      keys_init (void);