Implement ECS alternative for IDragDropOn and fully ECS disposal units (#6380)

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

Content.Shared/DragDrop/CanDragDropOnEvent.cs

@@ -0,0 +1,38 @@
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.DragDrop;
+
+/// <summary>
+/// Event that gets send to the target of a drag drop action
+/// Mark this event as handled to specify that the entity can be dropped on
+/// and set CanDrop to true or false, depending on whether dropping the entity onto the target is actually possible.
+/// </summary>
+public class CanDragDropOnEvent : HandledEntityEventArgs
+{
+    /// <summary>
+    ///     Entity doing the drag and drop.
+    /// </summary>
+    public EntityUid User { get; }
+
+    /// <summary>
+    ///     Entity that is being dragged.
+    /// </summary>
+    public EntityUid Dragged { get; }
+
+    /// <summary>
+    ///     Entity that is being dropped on.
+    /// </summary>
+    public EntityUid Target { get; }
+
+    /// <summary>
+    ///     If the dragged entity can be dropped on the target.
+    /// </summary>
+    public bool CanDrop { get; set; } = false;
+
+    public CanDragDropOnEvent(EntityUid user, EntityUid dragged, EntityUid target)
+    {
+        User = user;
+        Dragged = dragged;
+        Target = target;
+    }
+}

Content.Shared/DragDrop/SharedDragDropSystem.cs

@@ -0,0 +1,15 @@
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.DragDrop;
+
+public abstract class SharedDragDropSystem : EntitySystem
+{
+    protected bool? CheckDragDropOn(DragDropEvent eventArgs)
+    {
+        var canDragDropOnEvent = new CanDragDropOnEvent(eventArgs.User, eventArgs.Dragged, eventArgs.Target);
+
+        RaiseLocalEvent(eventArgs.Target, canDragDropOnEvent, false);
+
+        return canDragDropOnEvent.Handled ? canDragDropOnEvent.CanDrop : null;
+    }
+}