added stackable functionality to items and improved inventory management

src/de/miaurizius/jgame2d/entity/Entity.java

@@ -83,6 +83,8 @@ public class Entity {
     public int value;
     public int price;
     public int knockbackVal;
+    public boolean stackable;
+    public int amt = 1;
 
     public Entity(GamePanel panel) {
         this.panel = panel;
@@ -229,8 +231,8 @@ public class Entity {
             alive = false;
         }
     }
-    public void use(Entity entity) {
-
+    public boolean use(Entity entity) {
+        return false;
     } //If entity is consumable
     public void checkDrop() {