added stackable functionality to items and improved inventory management

src/de/miaurizius/jgame2d/entity/item/PotionObj.java

@@ -21,16 +21,17 @@ public class PotionObj extends Entity {
         description = "[" + name + "]\nHeals your life by " + value + ".";
 
         price = 50;
+        stackable = true;
     }
 
-    public void use(Entity entity) {
+    public boolean use(Entity entity) {
         panel.gameState = GameState.DIALOGUE;
         panel.ui.currentDialogue = "You drank a " + name + "!\n" +
                 "Your life has been recovered by " + value + ".";
         entity.life += value;
         if(panel.player.life > panel.player.maxLife) panel.player.life = panel.player.maxLife;
         panel.playSE(2);
-        panel.player.inventory.remove(this);
+        return true;
     }
 
 }