SVGHMI: add warning_labels, same as optional_labels but emits warnings at build time if element is missing.
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<init>
<xhtml:p><![CDATA[]]></xhtml:p>
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<cleanup>
<xhtml:p><![CDATA[]]></xhtml:p>
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<xhtml:p><![CDATA[]]></xhtml:p>
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<xhtml:p><![CDATA[]]></xhtml:p>
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