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.

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<?xml version='1.0' encoding='utf-8'?>

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<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="PYRO://127.0.0.1:61284">

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  <TargetType/>

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  <Libraries Enable_SVGHMI_Library="true"/>

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</BeremizRoot>