SVGHMI: Prepare accepting multiple clients on multiple HMI: Use POULibrary new capability to add variables to PLC Configuration. Now heartbeat and HMI tree root are added by the library part of SVGHMI, and not by each instance.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>