Add columns-reordered-dropped event for final column reorder completion

[CadenScharpf]

This PR introduces a columns-reordered-dropped event to trigger only once after the column is dropped into its final position. Unlike the existing events that fire during intermediate dragging, this event provides a cleaner, more efficient way to handle actions that depend on the final column order.

[AllanJard]

Hi - really sorry about the delay in replying here! My intention had been for columns-reordered to be used for this sort of thing - it might trigger a couple of times if the column is dragged over multiple locations, but it will always trigger after a change, meaning the UI will be in sync at that point. With the new dropped event, it only triggers if a mouse drag and drop is done - it doesn't happen after a state is reloaded via the API for example.

I'm a little hesitant for that reason - if someone were to listen for this event, there are other ways their columns could be reordered and the event wouldn't trigger.