Regression analysis: Cloning sheets failed on ctrlProps, i.e. checkboxes

and other GUI elements, add minimal support for these to allow clone to
succeed.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1730275 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/xssf/usermodel/XSSFRelation.java

@@ -344,6 +344,12 @@ public final class XSSFRelation extends POIXMLRelation {
             XSSFPivotCacheRecords.class
     );
 
+    public static final XSSFRelation CTRL_PROP_RECORDS = new XSSFRelation(
+            null,
+            "http://schemas.openxmlformats.org/officeDocument/2006/relationships/ctrlProp",
+            "/xl/ctrlProps/ctrlProp#.xml",
+            null
+    );
 
     private XSSFRelation(String type, String rel, String defaultName, Class<? extends POIXMLDocumentPart> cls) {
         super(type, rel, defaultName, cls);