SVGHMI: Lighter display of HMI Tree, no more icons and use buttons. Auto expand root. Fix loading of HMI tree XML backup (hmiclass attribute wasn't kept).

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>