Added drag and drop node connection re-wiring.
In the Node Graph, users can drag nodes onto existing connections to see automated re-wiring suggestions. Dropping the node into the chain will perform the re-wiring (which is undoable).

Also added GraphGadget::connectionGadgetAt as a convenience for connection queries.

Fixes issue #78.
Renaming members as m_dragReconnect
refactoring into private updateDragReconnectCandidate() to simplify dragMove() and reduce nesting
renaming connectionGadgetAtOrBelow to connectionGadgetAt and removing default value
merged dragDropConnections by fixing Changes