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.

README.md

@@ -86,6 +86,8 @@ GLFW bundles a number of dependencies in the `deps/` directory.
  - Added `GLFW_FOCUSED` window hint for controlling initial input focus
  - Added *partial and experimental* support for Wayland
  - Added *partial and experimental* support for Mir
+ - Changed the window states (focused, iconified and visible) to query the
+   system directly
  - Changed the default of `GLFW_REFRESH_RATE` to `GLFW_DONT_CARE` to maintain
    the default behavior
  - Changed static library to build as position independent code for easier use