Added optional queue system for input.

* Player.cs:
  Initial version.

* SpaceTapper.csproj, Difficulty.cs, GameDifficulty.cs, Difficulty.cs,
  DifficultySettings.cs, GameDifficulty.cs, DifficultySettings.cs:
  

* GameState.cs:
  Converted variables to use _ prefix.
  Start of player implementation.

* Game.cs:
  Added EndFrame event.
  Converted variables to use _ prefix.

* Input.cs:
  Added optional input queue.
  Documentation.

* MenuState.cs, DifficultyState.cs:
  Added handler for escape.
  Converted variables to use _ prefix.

* State.cs, ButtonList.cs:
  Converted variables to use _ prefix.

* Button.cs:
  Added rounding for the text origin, to avoid blurry fonts.