trigger: Add the ability to trigger data with an associated instance

trigger: Add the ability to trigger data with an associated instance

Right now triggers can only fire off handlers that have one piece
of data associated with them, but sometimes it's useful to have
an object associated with the data too.

This commit adds new api for adding "instance handlers" that have
an additional parameter and a new method `ply_trigger_set_instance`
that allows an object to be associated with the trigger and get
used for the additional parameter of the instance handlers.