Promote plugs to Box level

[johnhaddon]

We need a right-click plug menu for promoting plugs to the level of the enclosing box. Perhaps that should also be doable the other way around - adding a plug using an interface on the box.

Perhaps we should define a "user" CompoundPlug to ensure things are scoped in a sensible place.

The creation of customisable uis for the promoted plugs will be left to issue #25 - this ticket is just about getting the most basic functionality in at this point.