Made the Level of the ShopItem Update on the Shop When it is Purchased

- Made the ShopItem class implement the INotifyPropertyChanged interface
- Changed the LevelString property in ShopItem.cs to a getter and a setter so that it can utilize the INotifyPropertyChanged interface
- Created a method for setting the displayed level of the ShopItem: SetDisplayedLevel
- Used the SetCurrentLevel method for incrementing and decrementing the current level of a ShopItem
- Added a call to the SetDisplayedLevel method to each derived ShopItem's constructor. This was not done in the base class; otherwise, the max level for every ShopItem would be displayed as infinity
- Fixed an issue in the Shop in which the ShopItems under the "Game Saved!" animation would not work after the Save button was pressed once. Made the SaveGameAnimBorder not HitTestVisible to fix this
Merge branch 'ShopItemLevelUpdate' into MoreBalancing3