Create xbl.xml

Created XBL to test disabling command to disable buttons
MDN: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Commands#Example:_Toggling_command_disabled
SO: http://stackoverflow.com/questions/27454167/change-command-in-a-binding-using-xblinherits?noredirect=1#comment43349919_27454167