Click Drag Functionality + Refactor Interaction Interfaces (#1125)

Co-authored-by: Víctor Aguilera Puerto <6766154+Zumorica@users.noreply.github.com>
Co-authored-by: ComicIronic <comicironic@gmail.com>
Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Content.Server/Interfaces/GameObjects/Components/Interaction/IHandSelected.cs

@@ -0,0 +1,53 @@
+using System;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Interfaces.GameObjects;
+
+namespace Content.Server.Interfaces.GameObjects.Components.Interaction
+{
+    /// <summary>
+    ///     This interface gives components behavior when they're held on the selected hand.
+    /// </summary>
+    public interface IHandSelected
+    {
+        void HandSelected(HandSelectedEventArgs eventArgs);
+    }
+
+    public class HandSelectedEventArgs : EventArgs
+    {
+        public HandSelectedEventArgs(IEntity user)
+        {
+            User = user;
+        }
+
+        public IEntity User { get; }
+    }
+
+    /// <summary>
+    ///     Raised when an entity item in a hand is selected.
+    /// </summary>
+    [PublicAPI]
+    public class HandSelectedMessage : EntitySystemMessage
+    {
+        /// <summary>
+        ///     If this message has already been "handled" by a previous system.
+        /// </summary>
+        public bool Handled { get; set; }
+
+        /// <summary>
+        ///     Entity that owns the selected hand.
+        /// </summary>
+        public IEntity User { get; }
+
+        /// <summary>
+        ///     The item in question.
+        /// </summary>
+        public IEntity Item { get; }
+
+        public HandSelectedMessage(IEntity user, IEntity item)
+        {
+            User = user;
+            Item = item;
+        }
+    }
+}