Add ability to connect node inbetween two already connected nodes with drag and drop

[bentoogood]

Dragging a node over an existing connection should highlight the connection to indicate that insertion is possible.
Dropping the node whilst the connection is highlighted would perform the insertion.

This action reduces the number of clicks involved to make the connection, and makes insertion easier where the upstream or downstream connection is off screen somewhere.

this demo does not show highlighting before connecting

[johnhaddon]

I think we can achieve this as follows :

• During dragMove events, the GraphGadget does a selection step to find candidate connections
  • Drawing only the connections
  • Using a reasonably sized window around the cursor so you don't need to hit a connection exactly
• Candidate connections are then tested against the node being dragged, using Plug::acceptsInput() to see if an insertion is reasonable
• When a reasonable insertion is found, the GraphGadget stops drawing the original connection, and manually draws the proposed new connections.
• On dragEnd, the GraphGadget hooks things up