Rename appointment/event-related UI functions

* Rename "interaction.c" to "ui-day.c" since it no longer contains
  todo-related UI functions.

* Rename appointment/event-related UI functions to ui_day_*().

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

src/utils.c

@@ -105,7 +105,7 @@ void free_user_data(void)
   recur_apoint_llist_free();
   recur_event_llist_free();
   for (i = 0; i <= 37; i++)
-    interact_day_item_cut_free(i);
+    ui_day_item_cut_free(i);
   todo_free_list();
   notify_free_app();
 }