data tree FEATURE auto-delete for when-stmt

Still needs some changes in XPath processor to detect when a when-stmt
condition refers some other conditional nodes
diff --git a/src/libyang.h b/src/libyang.h
index 10d5883..d65071d 100644
--- a/src/libyang.h
+++ b/src/libyang.h
@@ -941,6 +941,7 @@
     LYVE_NOCOND,       /**< unsatisfied must/when condition (data) */
     LYVE_INORDER,      /**< invalid order of elements (data) */
     LYVE_INCOUNT,      /**< invalid number of elements (data) */
+    LYVE_INWHEN,       /**< irresolvable when condition (data) */
 
     LYVE_XPATH_INTOK,  /**< unexpected XPath token */
     LYVE_XPATH_EOF,    /**< unexpected end of an XPath expression */