Add keyboard layout support

This adds a keyboard layout switch callback and a query for the
human-readable name of the current layout.

Fixes #1201.

docs/input.dox

@@ -230,6 +230,32 @@ specified key is `GLFW_KEY_UNKNOWN` then the scancode is used, otherwise it is
 ignored.  This matches the behavior of the key callback, meaning the callback
 arguments can always be passed unmodified to this function.
 
+The name of a key will not change unless the keyboard layout changes.
+
+
+@subsection keyboard_layout Keyboard layout
+
+The human-readable name of the current keyboard layout is returned by @ref
+glfwGetKeyboardLayoutName.
+
+If you wish to be notified when the keyboard layout changes, set a keyboard
+layout callback.
+
+@code
+glfwSetKeyboardLayoutCallback(keyboard_layout_callback);
+@endcode
+
+The callback is called when the new layout takes effect for the application,
+which on some platforms may not happen until one of its windows gets input
+focus.
+
+@code
+void keyboard_layout_callback(void)
+{
+    update_keyboard_layout_name(glfwGetKeyboardLayoutName());
+}
+@endcode
+
 
 @section input_mouse Mouse input
 

docs/news.dox

@@ -9,6 +9,15 @@
 
 @subsection features_34 New features in version 3.4
 
+@subsubsection keyboard_layout_34 Keyboard layouts
+
+GLFW can now notify when the keyboard layout has changed with @ref
+glfwSetKeyboardLayoutCallback and provides the human-readable name of the
+current layout with @ref glfwGetKeyboardLayoutName.
+
+For more information, see @ref keyboard_layout.
+
+
 @subsubsection standard_cursors_34 More standard cursors
 
 GLFW now provides the standard cursor shapes @ref GLFW_RESIZE_NWSE_CURSOR and