Update the dragbar when the titlebar content size changes (#4845)

## Summary of the Pull Request

Add a `SizeChanged` handler to the titlebar content UI element. It's possible that this element's size will change after the dragbar's. When that happens, the drag bar won't send another `SizeChanged` event, because the dragbar's _size_ didn't change, only it's position.

## References

We originally duped this issue to #4166, but after #4829 fixed that issue, this one persisted. They're all related, and _look_ like dupes, but they weren't.

## PR Checklist
* [x] Closes #4288
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

## Validation Steps Performed

I had a solid 100% repro that doesn't repro anymore.

I've maximized, restored, resized, and generally played with the window a bunch.

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -110,6 +110,16 @@ void NonClientIslandWindow::SetContent(winrt::Windows::UI::Xaml::UIElement conte
 void NonClientIslandWindow::SetTitlebarContent(winrt::Windows::UI::Xaml::UIElement content)
 {
     _titlebar.Content(content);
+
+    // GH#4288 - add a SizeChanged handler to this content. It's possible that
+    // this element's size will change after the dragbar's. When that happens,
+    // the drag bar won't send another SizeChanged event, because the dragbar's
+    // _size_ didn't change, only it's position.
+    const auto fwe = content.try_as<winrt::Windows::UI::Xaml::FrameworkElement>();
+    if (fwe)
+    {
+        fwe.SizeChanged({ this, &NonClientIslandWindow::_OnDragBarSizeChanged });
+    }
 }
 
 // Method Description: