Overhauled several systems. Start of menu state.

* Entity.cs, Resource.cs, Button.cs, ButtonList.cs, Extensions.cs,
  MenuState.cs:
  Initial version.

* Input.cs:
  Now based off events.
  Added delegates to control processing of keys / mouse buttons.

* Game.cs:
  Added Font resource.
  Added Exit().

* Program.cs:
  Added default state.

* State.cs:
  Added input.