Fix editing amount of a transaction

Only validate split items if the SplitView is actually shown, which
makes the transaction a "split-transaction". Only then the original
amount cannot be different that the split-total.

Fixes #100

src/SplitView.cpp

@@ -325,12 +325,13 @@ SplitView::MessageReceived(BMessage* msg)
 			if (!fAmount->Validate())
 				break;
 
-			if (fSplitContainer->IsHidden())
+			if (fSplitContainer->IsHidden()) {
 				if (!fCategory->Validate())
 					break;
-
-			if (!ValidateSplitItems())
-				break;
+			} else {
+				if (!ValidateSplitItems())
+					break;
+			}
 			Account* account = fTransaction.GetAccount();
 			if (!account)
 				ShowBug("NULL transaction account in SplitView::M_ENTER_TRANSACTION");