SVGHMI: add support for multiline widget declaration using svg:desc.

Declaration started in inkscape:label and interupted by a '\' can be continued in the description field.
After an empty line, the rest of the description field is left for description text.

<?xml version='1.0' encoding='utf-8'?>
<BACnetServerNode xmlns:xsd="http://www.w3.org/2001/XMLSchema" UDP_Port_Number="47808" BACnet_Device_ID="3" BACnet_Communication_Control_Password=" " BACnet_Device_Location="Beremiz lab" Network_Interface="virbr0"/>