Added GrindsUnit trait for RA2:YR-style Grinder structure logic.

Added GrindsUnits trait:
        int RefundPercent
        bool RefundPassengers
        string GrindSound

Code style cleanup in Passenger.cs

Added bool DrawPips to SelectionDecorations.cs
        Need a way to show normal cursors (non-cargo)
        even if we have `Cargo:`

Test case in RA with POWR (will remove after testing):
        Accepts inf and vehicles (APCs),
        note: does not play nicely with EjectOnDeath

Also useful for #5910 as the Soviet (later Yuri) Cloning Vat had this feature for infantry.
We'd need a real use for this if we actually add it to the current mods so there is no reason for this to move around to different structures.