Remove pe::System and replace it with a single global sf::RenderWindow

pe::System is mostly just a wrapper for sf::RenderWindow.
Every time you want to use another feature of sf::RenderWindow, you
would have to add a wrapper function for it.

The only other useful functions of it were handling of some basic events
like window closed, and a key checking function.

I moved the handling of basic events to each State, and added a KeyState class
which is a wrapper for checking key states.

Having more than one window doesn't make sense for a game like Pickin' Sticks.
Having a single globally accessable renderwindow eliminates all of the
"haxxy manager passing" we had to do until now.