Merge bitcoin-core/gui#336: Do not exit and re-enter main event loop during shutdown

451ca244db qt, refactor: Drop intermediate BitcoinApplication::shutdownResult slot (Hennadii Stepanov)
f3a17bbe5f qt: Do not exit and re-enter main event loop during shutdown (Hennadii Stepanov)
b4e0d2c431 qt, refactor: Allocate SendConfirmationDialog instances on heap (Hennadii Stepanov)
332dea2852 qt, refactor: Keep HelpMessageDialog in the main event loop (Hennadii Stepanov)
c8bae37a7a qt, refactor: Keep PSBTOperationsDialog in the main event loop (Hennadii Stepanov)
7fa91e8312 qt, refactor: Keep AskPassphraseDialog in the main event loop (Hennadii Stepanov)
6f6fde30e7 qt, refactor: Keep EditAddressDialog in the main event loop (Hennadii Stepanov)
59f7ba4fd7 qt, refactor: Keep CoinControlDialog in the main event loop (Hennadii Stepanov)
7830cd0b35 qt, refactor: Keep OptionsDialog in the main event loop (Hennadii Stepanov)
13f618818d qt: Add GUIUtil::ShowModalDialogAndDeleteOnClose (Hennadii Stepanov)

Pull request description:

  On master (1ef34ee25e) during shutdown `QApplication` exits the main event loop, then re-enter again.

  This PR streamlines shutdown process by removing the need to interrupt the main event loop, that is required for #59.

  Also, blocking [`QDialog::exec()`](https://doc.qt.io/qt-5/qdialog.html#exec) calls are replaced with safer [`QDialog::show()`](https://doc.qt.io/qt-5/qwidget.html#show), except for `SendConfirmationDialog` as that change is not trivial (marked as TODO).

  The [`QDialog::open()`](https://doc.qt.io/qt-5/qdialog.html#open) was not used because the actual modality mode (application modal or window modal) of a dialog depends on whether it has a parent.

  This PR does not change behavior, and all touched dialogs are still application modal.
  As a follow up, a design research could suggest to make some dialogs window modal.

  NOTE for reviewers: quitting app while a dialog is open (e.g., via systray icon menu) must work fine.

ACKs for top commit:
  laanwj:
    Code review and lighly tested ACK 451ca244db
  promag:
    ACK 451ca244db, just changed signal to `quitRequested`.

Tree-SHA512: ef01ab6ed803b202e776019a4e1f592e816f7bc786e00574b25a0bf16be2374ddf9db21f0a26da08700df7ef0ab9e879550df46dcfe3b6d940f5ed02ca5f8447

src/qt/walletframe.cpp

@@ -221,10 +221,9 @@ void WalletFrame::gotoLoadPSBT(bool from_clipboard)
         return;
     }
 
-    PSBTOperationsDialog* dlg = new PSBTOperationsDialog(this, currentWalletModel(), clientModel);
+    auto dlg = new PSBTOperationsDialog(this, currentWalletModel(), clientModel);
     dlg->openWithPSBT(psbtx);
-    dlg->setAttribute(Qt::WA_DeleteOnClose);
-    dlg->exec();
+    GUIUtil::ShowModalDialogAndDeleteOnClose(dlg);
 }
 
 void WalletFrame::encryptWallet()