#192 Car Part / Item should support dummy locos

[Dave Bullis]

Currently, Car Prototype has an 'Is Loco?' toggle whose value is inherited by Car Part / Item.
However, Loco models can be dummies: externally a loco but unpowered.
This is a cheaper way to create Multiple Units with only one unit powered
Car Part / Item dialogs should also display 'Is Loco?' toggle which is disabled if the Prototype (for Part) or Prototype / Part (for Item) is not a Loco