Remove pe::KeyState and add keyDown() to pe::Application

pe::KeyState relied on sf::Keyboard::KeyCount to allocate memory for
holding the key states. It looks like KeyCount isn't reliable in that regard,
and it made the program crash.

Instead, pe::Application now has an isKeyDown method, which is
really a wrapper around sf::Keyboard::IsKeyPressed(), except it also
checks if the window has focus.