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.

docs/news.dox

@@ -32,6 +32,11 @@ GLFW now supports window maximization with @ref glfwMaximizeWindow and the
 [GLFW_MAXIMIZED](@ref window_attribs_wnd) window hint and attribute.
 
 
+@subsection news_32_focus Window input focus control
+
+GLFW now supports giving windows input focus with @ref glfwFocusWindow.
+
+
 @section news_31 New features in 3.1
 
 These are the release highlights.  For a full list of changes see the

docs/window.dox

@@ -715,6 +715,13 @@ int visible = glfwGetWindowAttrib(window, GLFW_VISIBLE);
 
 @subsection window_focus Window input focus
 
+Windows can be given input focus and brought to the front with @ref
+glfwFocusWindow.
+
+@code
+glfwFocusWindow(window);
+@endcode
+
 If you wish to be notified when a window gains or loses input focus, whether by
 the user, system or your own code, set a focus callback.