Created a mini icon for the castle, which can be displayed next to the castle's health or anywhere really

-Made ShopItems default to being upgradeable an infinite number of times and added a check for the infinite value when checking if a ShopItem can be bought
-Removed the MaxLevel const and the method CanUpgradeCastle in Player.cs since there is no longer a limit on how many times the player can upgrade the Castle
Created a 3-frame Goblin walking animation

-Changed MeleeEnemy to use the Goblin animation
-Disabled the player archer so we can easily see the animation without constant interruption
-Added reversible animation support to Animation.cs
Added the Shop Item Price to the Shop

-Created the PriceString property in ShopItem.cs for data binding the price of the item in XAML
-Changed the item template for Shop Items to include the price
Added the Mini Castle Icon to the Shop

-Added the Mini Castle Icon next to the player's Health Bar in the Shop
Improved how invincibility works across all objects

-Slightly improved the Goblin walking animation
-Added an InvincibleSheet Texture2D field to LevelObject.cs, which is an invincible version of the object
  -The same animations can be used, so all you need to do is draw this Texture2D with a higher depth value than the ObjectSheet (so it shows up)
Merge branch 'BetterInvincibility' into AddCastleIconToShop
Merge branch 'AddCastleIconToShop'