Drag offset is now passed explicitly to the dragging callback so that the user can choose what to do with it.
Extracted a function to hit test a connector.
Moved the dragging connection to a group, the visibility of which (only when dragging out a connection) is controlled as a unit.
The connection end points are now a permanent part of the 'dragging connection' (rather than just a debug aid) as they look like what I had in the original WPF version of the flowchart control.  They have proper CSS classes defined now.