Add cost difference from previous vessel(s) while editing a vessel

[Clayell]

If this actually works, it will show how much more the new edited vessel will cost, compared to the previous unedited vessel(s).
Fixes #2072
(this is kinda dumb tbh)

[siimav]

What is this? Vessel merge feature is no longer used.

[Clayell]

Oh yea, you're right, I totally forgot. I tried taking this from:

RP-1/Source/RP0/Utilities/KCTUtilities.cs

Lines 456 to 460 in 0a7d57a

	foreach (VesselProject v in SpaceCenterManagement.Instance.MergedVessels)
	{
	usedShipsCost += v.GetTotalCost();
	v.RemoveFromBuildList(out _);
	}

Is that code redundant now? How would I get the cost of the vessel before edits? (assuming that editedVessel.effectiveCost is the cost of the vessel after edits)