Double middle click on taskbar preview closes application (#7871)

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## Summary of the Pull Request
A second close command (middle click on taskbar preview) overrides the warning dialog and closes the application.

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## References

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## PR Checklist
* [x] Closes #7451 
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

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## Detailed Description of the Pull Request / Additional comments
When a close command is invoked (middle click on taskbar preview or 'X' button), a new flag is set. When the user wants to close again (this time only via the taskbar preview, as the 'X' button is disabled), the application is closed. If the user cancels the dialog, the flag is reset to prevent accidental closing on a subsequent close command.

<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
I am developing with a [Windows 10 virtual machine](https://developer.microsoft.com/en-us/windows/downloads/virtual-machines/) provided by Microsoft. I tested manually. I considered the 'X' button, middle click on taskbar preview, and Alt+F4. Only a middle click on the taskbar preview does override the dialog.

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1388,8 +1388,9 @@ namespace winrt::TerminalApp::implementation
     //   than one tab opened, show a warning dialog.
     void TerminalPage::CloseWindow()
     {
-        if (_tabs.Size() > 1 && _settings.GlobalSettings().ConfirmCloseAllTabs())
+        if (_tabs.Size() > 1 && _settings.GlobalSettings().ConfirmCloseAllTabs() && !_displayingCloseDialog)
         {
+            _displayingCloseDialog = true;
             _ShowCloseWarningDialog();
         }
         else
@@ -2055,6 +2056,20 @@ namespace winrt::TerminalApp::implementation
         _CloseAllTabs();
     }
 
+    // Method Description:
+    // - Called when the close button of the content dialog is clicked.
+    //   This resets the flag _displayingCloseDialog, which was set before
+    //   opening the dialog. Otherwise, the Terminal app would be closed
+    //   on the next close request without showing the warning dialog.
+    // Arguments:
+    // - sender: unused
+    // - ContentDialogButtonClickEventArgs: unused
+    void TerminalPage::_CloseWarningCloseButtonOnClick(WUX::Controls::ContentDialog /* sender */,
+                                                       WUX::Controls::ContentDialogButtonClickEventArgs /* eventArgs*/)
+    {
+        _displayingCloseDialog = false;
+    }
+
     // Method Description:
     // - Hook up keybindings, and refresh the UI of the terminal.
     //   This includes update the settings of all the tabs according

src/cascadia/TerminalApp/TerminalPage.h

@@ -135,10 +135,12 @@ namespace winrt::TerminalApp::implementation
         void _CreateNewTabFromSettings(GUID profileGuid, TerminalApp::TerminalSettings settings);
         winrt::Microsoft::Terminal::TerminalConnection::ITerminalConnection _CreateConnectionFromSettings(GUID profileGuid, TerminalApp::TerminalSettings settings);
 
+        bool _displayingCloseDialog{ false };
         void _SettingsButtonOnClick(const IInspectable& sender, const Windows::UI::Xaml::RoutedEventArgs& eventArgs);
         void _FeedbackButtonOnClick(const IInspectable& sender, const Windows::UI::Xaml::RoutedEventArgs& eventArgs);
         void _AboutButtonOnClick(const IInspectable& sender, const Windows::UI::Xaml::RoutedEventArgs& eventArgs);
         void _CloseWarningPrimaryButtonOnClick(Windows::UI::Xaml::Controls::ContentDialog sender, Windows::UI::Xaml::Controls::ContentDialogButtonClickEventArgs eventArgs);
+        void _CloseWarningCloseButtonOnClick(Windows::UI::Xaml::Controls::ContentDialog sender, Windows::UI::Xaml::Controls::ContentDialogButtonClickEventArgs eventArgs);
         void _ThirdPartyNoticesOnClick(const IInspectable& sender, const Windows::UI::Xaml::RoutedEventArgs& eventArgs);
 
         void _HookupKeyBindings(const Microsoft::Terminal::Settings::Model::KeyMapping& keymap) noexcept;

src/cascadia/TerminalApp/TerminalPage.xaml

@@ -51,7 +51,8 @@ the MIT License. See LICENSE in the project root for license information. -->
             x:Name="CloseAllDialog"
             x:Uid="CloseAllDialog"
             DefaultButton="Primary"
-            PrimaryButtonClick="_CloseWarningPrimaryButtonOnClick">
+            PrimaryButtonClick="_CloseWarningPrimaryButtonOnClick"
+            CloseButtonClick="_CloseWarningCloseButtonOnClick">
         </ContentDialog>
 
         <ContentDialog