gui: fix crash on selecting "Mask values" in transaction view

This commits fixes a crash bug that can be caused with the following steps:
- change to the "Transactions" view
- right-click on an arbitrary transaction -> "Show transaction details"
- close the transaction detail window again
- select "Settings" -> "Mask values"

The problem is that the list of opened dialogs, tracked in the member
variable `m_opened_dialogs`, is only ever appended with newly opened
transaction detail dialog pointers, but never removed. This leads to
dangling pointers in the list, and if the "Mask values" menu item is
selected, a crash is caused in the course of trying to close the opened
transaction detail dialogs (see `closeOpenedDialogs()` method). Fix this
by removing the pointer from the list if the corresponding widget is
destroyed.

Github-Pull: https://github.com/bitcoin-core/gui/pull/774
Rebased-From: e26e665f9f

src/qt/transactionview.cpp

@@ -532,6 +532,9 @@ void TransactionView::showDetails()
         TransactionDescDialog *dlg = new TransactionDescDialog(selection.at(0));
         dlg->setAttribute(Qt::WA_DeleteOnClose);
         m_opened_dialogs.append(dlg);
+        connect(dlg, &QObject::destroyed, [this, dlg] {
+            m_opened_dialogs.removeOne(dlg);
+        });
         dlg->show();
     }
 }