Add Meta Key mapping to xtscancodes.js

OSLeft/OSRight have been renamed to MetaLeft/MetaRight respectively.

This change will allow the Super/Windows key to be used under the qemu rfb extension under Chrome and updated browsers.

https://github.com/w3c/uievents-code/issues/7
https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/code

include/xtscancodes.js

@@ -110,6 +110,8 @@ XtScancode["ArrowDown"] = 0xE050;
 XtScancode["PageDown"] = 0xE051;
 XtScancode["Insert"] = 0xE052;
 XtScancode["Delete"] = 0xE053;
+XtScancode["MetaLeft"] = 0xE05B;
+XtScancode["MetaRight"] = 0xE05C;
 XtScancode["OSLeft"] = 0xE05B;
 XtScancode["OSRight"] = 0xE05C;
 XtScancode["ContextMenu"] = 0xE05D;