Expose Offset to DateTime conversion

[TruFelix]

At the moment the functionality to convert a Offset to the appropriate DateTime when dragging Events (using PartDayDraggableEvent) is not exposed, _DragInfos is private. It would be beneficial to expose this functionality, to allow for a better customization and handling custom drag'n'drop

Possible Solution: It would be enough to just expose the _DragInfos class, by doing so, you could just call DragInfos.resolveOffset(...) inside of a custom Draggable. Also it would be great, to have the possibility to specify, which DragInfos instance should be used to be able to call this functionality from another subtree.