add method to check if an open transaction is empty

This is needed to avoid to incorrectly abort a transaction if a recompute was done between opening and closing an editor in the property view

src/App/Document.h

@@ -303,6 +303,9 @@ public:
     void abortTransaction();
     /// Check if a transaction is open
     bool hasPendingTransaction() const;
+    /// Check if a transaction is open and its list is empty.
+    /// If no transaction is open true is returned.
+    bool isTransactionEmpty() const;
     /// Set the Undo limit in Byte!
     void setUndoLimit(unsigned int UndoMemSize=0);
     /// Returns the actual memory consumption of the Undo redo stuff.