Unify key name string handling

This makes key names per-key static strings for all supported platforms.

Fixes #1200.

include/GLFW/glfw3.h

@@ -4190,9 +4190,11 @@ GLFWAPI int glfwRawMouseMotionSupported(void);
  *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
  *  GLFW_PLATFORM_ERROR.
  *
+ *  @remark The contents of the returned string may change when a keyboard
+ *  layout change event is received.
+ *
  *  @pointer_lifetime The returned string is allocated and freed by GLFW.  You
- *  should not free it yourself.  It is valid until the next call to @ref
- *  glfwGetKeyName, or until the library is terminated.
+ *  should not free it yourself.  It is valid until the library is terminated.
  *
  *  @thread_safety This function must only be called from the main thread.
  *