Add glfwFocusWindow

This removes the (undocumented) behavior where glfwShowWindow would
bring the window to front and set input focus.  That function now
does what it says.

include/GLFW/glfw3.h

@@ -2283,6 +2283,34 @@ GLFWAPI void glfwShowWindow(GLFWwindow* window);
  */
 GLFWAPI void glfwHideWindow(GLFWwindow* window);
 
+/*! @brief Brings the specified window to front and sets input focus.
+ *
+ *  This function brings the specified window to front and sets input focus.
+ *  The window should already be visible and not iconified.
+ *
+ *  By default, both windowed and full screen mode windows are focused when
+ *  initially created.  Set the [GLFW_FOCUSED](@ref window_hints_wnd) to disable
+ *  this behavior.
+ *
+ *  __Do not use this function__ to steal focus from other applications unless
+ *  you are certain that is what the user wants.  Focus stealing can be
+ *  extremely disruptive.
+ *
+ *  @param[in] window The window to give input focus.
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
+ *  GLFW_PLATFORM_ERROR.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref window_focus
+ *
+ *  @since Added in version 3.2.
+ *
+ *  @ingroup window
+ */
+GLFWAPI void glfwFocusWindow(GLFWwindow* window);
+
 /*! @brief Returns the monitor that the window uses for full screen mode.
  *
  *  This function returns the handle of the monitor that the specified window is