qt: Make it possible to show details for multiple transactions

A small GUI annoyance for me has always been that it's impossible to
have multiple transaction detail windows open, for example to compare
transactions.

This patch makes the window non-modal so that it is possible to open
transaction details at will.

src/qt/transactionview.cpp

@@ -478,8 +478,9 @@ void TransactionView::showDetails()
     QModelIndexList selection = transactionView->selectionModel()->selectedRows();
     if(!selection.isEmpty())
     {
-        TransactionDescDialog dlg(selection.at(0));
+        TransactionDescDialog *dlg = new TransactionDescDialog(selection.at(0));
-        dlg.exec();
+        dlg->setAttribute(Qt::WA_DeleteOnClose);
+        dlg->show();
     }
 }