Made Invincibility Able to be Purchased More Than Once

- When the player purchases and uses the Invincibility consumable, he/she would not be able to purchase another one. Fixed this by decrementing the CurLevel of the Invincibility ShopItem by 1 when the item is used in-game
Balanced the game and slighly changed enemy spawning again

-If more than one enemy spawns at a time, the consecutive enemies are moved further to the left to avoid overlap
-Changed the MinSpeedIncrease in EnemySpawning from 14 to 20
-Fixed 5 enemies spawning at later levels by reducing the max # of additional enemies that can spawn from 4 to 3
  -Also made it so more enemies can spawn every 8 levels instead of every 6 levels
-Reduced the duration increase of subsequent levels (levels after Level 1) to 1000 from 1100

-Changed shop items:
  -Slowers cost 10000 Gold instead of 15000
  -Slower Upgrades cost 7500 Gold instead of 12500
  -Archer Upgrades cost 12500 Gold instead of 15000
  -FortifyCastle now can be upgraded 5 times, decreasing damage dealt by enemies by 3% each time and maxing out at 15%. It costs 5000 Gold each upgrade, maxing out at 25000

-Renamed the Player's FortifyCastle method to StrengthenCastle and changed how it works to accomodate the multiple upgrades of FortifyCastle
Even more balancing!

-Fixed the animation slow amount calculation in Enemy.ApplySlow to be lower and more consistent with slower level
-Reduced the base speed of Flying and Ranged Goblins by .5
-Made it so the Player cannot attack before enemies start spawning
-Made projectiles give Gold when attacked and fade out just like enemies

-Reduced the price of BuyArcher from 20000 Gold to 15000 Gold
-Reduced the price of UpgradeArcher from 12500 Gold to 10000 Gold

-Made it so enemy invincibility scaled based off their bonus speed, not the current level
-Capped bonus speed at 6
Started Redesigning the Shop

- Changed the ItemTemplate of the three ShopItemsList GridViews on the Shop. They now include the current and max level of the shop item, and the Buttons containing the shop item images no longer have a fixed width and height
- Changed the ItemsPanelTemplate of the GridView to a VirtualizingStackPanel from a VariableSizedWrapGrid so that the full contents of each GridViewItem would be displayed
- Made the first two rows of the Shop scale to different screen sizes and resolutions
- Changed the headings of each section of the Shop in which the player can purchase items
- Moved the RepairWalls item to the "Fort" section of the Shop
- Fixed a small bug in which the "Continue Game" button would still be disabled if the player saved the game in the Shop and then returned to the Title Screen from the Pause Menu
Merge remote-tracking branch 'origin/ShopRedesign' into MoreBalancing2
Merge branch 'MoreBalancing2'