added cost amount to items

added Shop UI
added Shop.cs
 * you now can buy items and you lose the cost when bought
 * shop is scrollable

added comments in Inventory.cs, InventorySlot.cs, InventoryUI.cs
now showing description when hovering over items in shop or inventory

Assets/Scripts/Shop.cs

@@ -0,0 +1,60 @@
+using System.Collections.Generic;
+using UnityEngine;
+
+public class Shop : MonoBehaviour {
+    #region Singleton
+
+    public static Shop instance;
+
+    private void Awake() {
+        if(instance != null) {
+            Debug.LogWarning("More than one instance of Shop found");
+        }
+
+        instance = this;
+    }
+
+    #endregion
+    
+    public Dictionary<Item, int> items;
+    public Item[] tempItems;
+    
+    /**
+     * 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 tempItems) {
+            AddItem(item, 1);
+        }
+    }
+    
+    /**
+     * Adds the specified amount of items to the Shop
+     */
+    public void AddItem(Item item, int amount) {
+        if(!items.ContainsKey(item)) {
+            items.Add(item, amount);
+        } else {
+            items[item] += amount;
+        }
+
+        onItemChangedCallback?.Invoke();
+    }
+    // TODO: add to buy more than one item
+    /**
+     * Removes the specified amount of items in the Shop
+     */
+    public void RemoveItem(Item item, int amount) {
+        if(items[item] <= 0) {
+            items.Remove(item);
+        } else {
+            items[item] -= amount;
+        }
+
+        onItemChangedCallback?.Invoke();
+    }
+}