This fixes ttp 331811 - The control+left drag click functionality in the material editor broke recently.
I've changed the preview connector pins to be stored as a handle (NodeGuid + PinName) so that we can reference them persistently across rebuilds of the graph
Reviewed by Matthew Griffin, Matt Kuhlenschmidt
[CL 2056263 by Andrew Rodham in Main branch]
#proj core
#branch UE4
#summary Removed header groups, removed some redundant includes and replaced with forward declarations. Fixed compilation errors.
[CL 2055361 by Mikolaj Sieluzycki in Main branch]
#proj core
#branch UE4
#summary Removed header groups, removed some redundant includes and replaced with forward declarations. Fixed compilation errors.
[CL 2054304 by Mikolaj Sieluzycki in Main branch]
#summary Replaced instances of DragDrop::IsTypeMatch with FDragDropEvent::GetOperationAs()
#note Please change any instances of DragDrop::IsTypeMatch for the templated FDragDropEvent::GetOperationAs<>() or FDragDropOperation::IsOfType<>() where necessary
#proj Editor
#branch UE4
#reviewedby Nick.Atamas
[CL 2042222 by Andrew Rodham in Main branch]
