Add GLFW_HOVERED for polling cursor hover state

This window attribute corresponds to the cursor enter/leave callback.

Fixes #1166.

include/GLFW/glfw3.h

@@ -814,6 +814,7 @@ extern "C" {
  *  [window attribute](@ref GLFW_TRANSPARENT_FRAMEBUFFER_attrib).
  */
 #define GLFW_TRANSPARENT_FRAMEBUFFER 0x0002000A
+#define GLFW_HOVERED                0x0002000B
 
 /*! @brief Framebuffer bit depth hint.
  *