Changing SDL2_KeyboardUtil to cast to int to remove boxing

Changing SDL2_KeyboardUtil to cast to int for the dictionary to avoid boxing which contributes to garbage collections. This is based on Shawn's comment in [his blog](http://blogs.msdn.com/b/shawnhar/archive/2007/07/02/twin-paths-to-garbage-collector-nirvana.aspx):

> If you use an enum type as a dictionary key, internal dictionary operations will cause boxing. You can avoid this by using integer keys, and casting your enum values to ints before adding them to the dictionary.