Bind the callback func to the event sequence for all items given by tagOrId, so that func is invoked whenever that event occurs for one of the items. This is like Widget.bind but operates on canvas items rather than on whole widgets; only mouse, keyboard and virtual events may be bound. Mouse events are directed to the current item and keyboard events to the focus item (see focus()). If add is true the new binding is added to any existing bindings for the same sequence, rather than replacing them. Return the identifier of the bound function, which can be passed to tag_unbind().