[Input] Add IsAnyKey/ButtonDown to Keyboard/MouseState.

Adds properties to KeyboardState and MouseState to see if any key or button is
down. This should be faster than iterating over all the public IsDown
properties as we can make use of the internal bit fields.

Request from a [twitter conversation](https://twitter.com/Frassle/status/534817689945255937) with @Phrohdoh.