Switch Manager: Add support for additional sources

[freezy]

We'll have game mechanics like troughs that have multiple switches (and coils). I think the best way to support them without making everything incredibly complicated is to interpret them as "source".

I would imagine having an interface like that:

public interface ISwitchDevice
{
    string Name { get; }
    IEnumerable<GamelogicEngineSwitch> Switches;
}

Our trough component would then implement this interface, and it thus appear in the switch manager as additional source:

Data-wise, we have two possibilities:

1. Change Source from int to string, and introduce "special" values for input system, playfield, constant (maybe prefix with double underscore?)
2. Add an other DeviceSource field that is a string, and add another Source value (e.g. -1 = use DeviceSource)

I would prefer 2. since it's more proper. We also need to store the actual value in a new field, let's say DeviceItem.

So for the trough example above, we would have the following data:

• Id = "15"
• Description = "Trough I"
• Source = SwitchSource.Device
• DeviceSource = "Trough"
• DeviceItem = "Switch I"
• Type = SwitchType.OnOff