Add glfwGetKeyName

Fixes #464.

docs/input.dox

@@ -192,6 +192,22 @@ void charmods_callback(GLFWwindow* window, unsigned int codepoint, int mods)
 @endcode
 
 
+@subsection input_key_name Key names
+
+If you wish to refer to keys by name, you can query the keyboard layout
+dependent name of printable keys with @ref glfwGetKeyName.
+
+@code
+const char* key_name = glfwGetKeyName(GLFW_KEY_W, 0);
+show_tutorial_hint("Press %s to move forward", key_name);
+@endcode
+
+This function can handle both [keys and scancodes](@ref input_key).  If the
+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.
+
+
 @section input_mouse Mouse input
 
 Mouse input comes in many forms, including cursor motion, button presses and