Allow cancelling of a split paste

Currently when you paste a split you can not cancel the changes as they
are already committed. By opening the transaction for editing before
the split paste the cancel option is now available.

gnucash/register/ledger-core/split-register.c

@@ -953,6 +953,10 @@ gnc_split_register_paste_current (SplitRegister *reg)
             }
         }
 
+        /* Open the transaction for editing. */
+        if (gnc_split_register_begin_edit_or_warn (info, trans))
+        {
+            LEAVE("can't begin editing");
             return;
         }