Add interact_day_item_copy()

This can be used to copy an item, so that it can be pasted somewhere
else later.

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

src/calcurse.h

@@ -723,6 +723,7 @@ void interact_day_item_pipe(void);
 void interact_day_item_repeat(void);
 void interact_day_item_cut_free();
 void interact_day_item_cut(unsigned *, unsigned *);
+void interact_day_item_copy(unsigned *, unsigned *);
 void interact_day_item_paste(unsigned *, unsigned *);
 void interact_todo_add(void);
 void interact_todo_delete(void);