Storage system refactor & map init.

* Demonstrated map init working with guns, toolboxes, tool lockers.
* Refactored EntityStorage and ServerStorage to have a common interface.
* EntityStorage no longer uses ServerStorage PURELY for visuals.
  Use an appearance visualizer instead.

Content.Client/GameObjects/Components/Storage/StorageVisualizer2D.cs

@@ -0,0 +1,47 @@
+using Content.Shared.GameObjects.Components.Storage;
+using Robust.Client.GameObjects;
+using Robust.Client.Interfaces.GameObjects.Components;
+using Robust.Shared.Utility;
+using YamlDotNet.RepresentationModel;
+
+namespace Content.Client.GameObjects.Components.Storage
+{
+    public sealed class StorageVisualizer2D : AppearanceVisualizer
+    {
+        private string _stateOpen;
+        private string _stateClosed;
+
+        public override void LoadData(YamlMappingNode node)
+        {
+            base.LoadData(node);
+
+            if (node.TryGetNode("state_open", out var child))
+            {
+                _stateOpen = child.AsString();
+            }
+
+            if (node.TryGetNode("state_closed", out child))
+            {
+                _stateClosed = child.AsString();
+            }
+        }
+
+        public override void OnChangeData(AppearanceComponent component)
+        {
+            base.OnChangeData(component);
+
+            if (!component.Owner.TryGetComponent(out ISpriteComponent sprite))
+            {
+                return;
+            }
+
+            component.TryGetData(StorageVisuals.Open, out bool open);
+            sprite.LayerSetState(StorageVisualLayers.Door, open ? _stateOpen : _stateClosed);
+        }
+    }
+
+    public enum StorageVisualLayers
+    {
+        Door
+    }
+}