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/ShopSlot.cs

@@ -0,0 +1,93 @@
+using System;
+using System.Collections;
+using TMPro;
+using UnityEngine;
+using UnityEngine.EventSystems;
+using UnityEngine.UI;
+
+public class ShopSlot : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler {
+    public Image icon;
+    public Item item;
+    public TextMeshProUGUI nameText;
+    public TextMeshProUGUI costText;
+    public TextMeshProUGUI amountText;
+    
+    private Shop _shop;
+    private Inventory _inventory;
+    private PlayerController _playerController;
+    
+    #region DescriptionHover
+
+    public float timeToWait;
+
+    public void OnPointerEnter(PointerEventData eventData) {
+        StopAllCoroutines();
+        StartCoroutine(StartTimer());
+    }
+
+    public void OnPointerExit(PointerEventData eventData) {
+        StopAllCoroutines();
+        HoverManager.onMouseExit();
+    }
+
+    private void ShowMessage() {
+        if(item) {
+            HoverManager.onMouseHover(item.description, Input.mousePosition);
+        }
+    }
+
+    private IEnumerator StartTimer() {
+        yield return new WaitForSeconds(timeToWait);
+
+        ShowMessage();
+    }
+
+    #endregion
+
+    private void Start() {
+        _shop = Shop.instance;
+        _inventory = Inventory.instance;
+        _playerController = PlayerController.instance;
+    }
+
+    /**
+     * Sets the Item of the Shop Slot
+     */
+    public void AddItem(Item newItem) {
+        item = newItem;
+
+        icon.sprite = item.defaultSprite;
+        icon.enabled = true;
+    }
+    
+    /**
+     * Clears the Shop Slot
+     */
+    public void ClearSlot() {
+        item = null;
+        icon.sprite = null;
+        icon.enabled = false;
+        nameText.text = "";
+        costText.text = "";
+        amountText.text = "";
+    }
+
+    /**
+     * Gets called when the Shop Slot is clicked
+     */
+    public void UseItem() {
+        if(_playerController.money >= item.cost) {
+            _inventory.AddItem(item, 1);
+            _shop.RemoveItem(item, 1);
+            _playerController.money -= item.cost;
+            
+            Debug.Log("Buying Item: " + item.displayName);
+            Debug.Log("money left: " + _playerController.money);
+        } else {
+            Debug.Log("Not enough money to buy item.");
+        }
+        
+        _shop.onItemChangedCallback?.Invoke();
+        _inventory.onItemChangedCallback?.Invoke();
+    }
+}