Changed state attribs to direct access.
Changed the window states (focused, iconified and visible) to query the system directly. THIS IS A BREAKING CHANGE, although a fairly obscure one. It affects applications that both care about telling actual key events from synthetic ones, and are implemented in a non-self-synchronizing way, and that poll the GLFW_FOCUSED window attribute instead of using the window focus callback. If you maintain one of these, please feel free to drop me an email and I'll help any way I can to transition your application to 3.1. Fixes #189. Fixes #204.