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.