Adds disposal mailing units (again) (#7630)

Content.Shared/Disposal/MailingUnitUiMessages.cs

@@ -0,0 +1,23 @@
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Disposal;
+
+[Serializable, NetSerializable]
+public enum MailingUnitUiKey : byte
+{
+    Key
+}
+
+/// <summary>
+///     Message data sent from client to server when a disposal unit ui button is pressed.
+/// </summary>
+[Serializable, NetSerializable]
+public sealed class TargetSelectedMessage : BoundUserInterfaceMessage
+{
+    public readonly string? target;
+
+    public TargetSelectedMessage(string? target)
+    {
+        this.target = target;
+    }
+}