Add interlocking airlocks (#14177)

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/MachineLinking/Components/SignalTransmitterComponent.cs

@@ -1,3 +1,4 @@
+using Content.Server.MachineLinking.Events;
 using Content.Server.MachineLinking.System;
 
 namespace Content.Server.MachineLinking.Components
@@ -31,6 +32,13 @@ namespace Content.Server.MachineLinking.Components
         [ViewVariables(VVAccess.ReadWrite)]
         public float TransmissionRange = 30f;
 
+        /*
+         * Remember last output state to avoid re-raising a SignalChangedEvent if the signal
+         * level hasn't actually changed.
+         */
+        [ViewVariables(VVAccess.ReadWrite)]
+        public SignalState LastState = SignalState.Low;
+
         [DataField("outputs")]
         [Access(typeof(SignalLinkerSystem), Other = AccessPermissions.ReadExecute)] // FIXME Friends
         public Dictionary<string, List<PortIdentifier>> Outputs = new();