Super key modifier patch

This is just a quick patch to add GLFW_MOD_SUPER to Windows, OS X, and Linux (X11). On Windows it's the Windows keys, OS X the command key, and in the X11 implementation it uses Mod4Mask. Not 100% certain about the use of Mod4Mask, but it seems like the most likely candidate if going by Google and my VM.

There's also a check in the X11 key-down code to set the super modifier if either super key is pressed as well, as it seems sensible enough that if the super key is pressed, its modifier is itself.