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.
Fixed SceneInspector update.

Previously it made no attempt at all to update when a plug it was inspecting was dirtied. Now it does.

Fixes #176.
Reordered a conditional in the MultiSelectionMenu so that the name displayed when only
one selection is avaliable is the selection and not "all".
Merge pull request #181 from goddardl/MultiSelectionMenuFix

Small MultiSelectionMenu fix.
Improved SceneInspector shader representation.

It now displays the name of the terminating shader in the network rather than an obscure "ObjectVector..." string.

Fixes #147.
Removing todo.
Improved SceneInspector numeric formatting.

It now matches the formatting performed by the NumericWidget.

Fixes #88.
Merge pull request #182 from johnhaddon/sceneInspectorImprovements

Scene inspector improvements
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
Adding test case that doesn't show what I hoped it might.