Add function item:is_being_used()
Add function item:is_being_used(), returning a boolean, so we can know if an item is being used. The return value will be set to true after the event on_started, and to false after the event on_finished.
In my opinion, this may be quite useful (to make the code cleaner) when using combined custom items, like sword + feather, boots + feather, boots + sword, etc.