Improved SceneHierarchy to view any output ScenePlug, regardless of name.

This improves compatibility with Boxes, where the user can make an output plug with any name they want.
Prevented PlugValueWidget creation for all ScenePlugs.

Prior to this, we were only preventing their creation for the "in" and "out" plugs on SceneNodes. We now prevent their creation anywhere, meaning that they are no longer created incorrectly for outputs or inputs to Boxes.
Made BlockedConnection and UndoContext non-copyable.

They don't have any sensible copy semantics, so removing the copy constructor removes a trap for people to fall into.
WIP