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/ShopUI.cs

@@ -0,0 +1,57 @@
+using System.Linq;
+using UnityEngine;
+
+public class ShopUI : MonoBehaviour {
+    public Transform itemsParent;
+    public GameObject shopUI;
+    private Shop _shop;
+    private ShopSlot[] _slots;
+
+    private void Start() {
+        // Get Shop instance and add UpdateUI method to OnItemChanged delegate
+        _shop = Shop.instance;
+        _shop.onItemChangedCallback += UpdateUI;
+
+        // Add all ShopSlot GameObjects to _slots and turn off the Shop UI
+        _slots = itemsParent.GetComponentsInChildren<ShopSlot>();
+        ToggleShop();
+
+        // Set the icon to not be a raycast target for the Description Hovering to work
+        foreach(ShopSlot slot in _slots) {
+            slot.icon.raycastTarget = false;
+        }
+        
+        UpdateUI();
+    }
+
+    private void Update() {
+        // When "Shop" button is pressed turn on/off Shop UI
+        if(Input.GetButtonDown("Shop")) {
+            ToggleShop();
+        }
+    }
+
+    /**
+     * Turn on/off the Shop UI
+     */
+    private void ToggleShop() {
+        shopUI.SetActive(!shopUI.activeSelf);
+    }
+
+    /**
+     * Is called when something in the Shop UI should update
+     */
+    private void UpdateUI() {
+        // Add all items to the correct slots and clear the ones where no item should be
+        for(int i = 0; i < _slots.Length; i++) {
+            if(i < _shop.items.Count) {
+                _slots[i].AddItem(_shop.items.ElementAt(i).Key);
+                _slots[i].nameText.text = _slots[i].item.displayName;
+                _slots[i].costText.text = _slots[i].item.cost + " €";
+                _slots[i].amountText.text = _shop.items[_shop.items.ElementAt(i).Key] + " #";
+            } else {
+                _slots[i].ClearSlot();
+            }
+        }
+    }
+}