[DragDrop] Make use of SendEvent and user events, along with signal properties

[ivan-mogilko]

AGS 3.6.2 introduced SendEvent function, which also allows to send custom events into on_event callback.
Consider using these in DragDrop module, along with the signal properties.
Analyze if it would be convenient to deprecate signal properties when module is used in 3.6.2+ editor.