added ItemStorage.cs, ItemStorageSlot.cs, ItemStorageUI.cs

* Inventory, InventorySlot, InventoryUI, Shop, ShopSlot, ShopUI are extending them

Assets/Scripts/ItemStorage.cs

@@ -0,0 +1,46 @@
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ItemStorage : MonoBehaviour {
+    public Dictionary<Item, int> items;
+    public Item[] startItems;
+
+    /**
+     * Methods can be added to this and they will get called every time onItemChangedCallback gets Invoked
+     */
+    public delegate void OnItemChanged();
+    public OnItemChanged onItemChangedCallback;
+
+    private void Start() {
+        items ??= new Dictionary<Item, int>();
+        foreach(Item item in startItems) {
+            AddItem(item, 1);
+        }
+    }
+
+    /**
+     * Adds the specified amount of items to the Item Storage
+     */
+    public virtual void AddItem(Item item, int amount) {
+        if(!items.ContainsKey(item)) {
+            items.Add(item, amount);
+        } else {
+            items[item] += amount;
+        }
+
+        onItemChangedCallback?.Invoke();
+    }
+
+    /**
+     * Removes the specified amount of items in the Item Storage
+     */
+    public void RemoveItem(Item item, int amount) {
+        if(items[item] <= 0) {
+            items.Remove(item);
+        } else {
+            items[item] -= amount;
+        }
+
+        onItemChangedCallback?.Invoke();
+    }
+}