Fix dialog losing focus when changing WindowState

[corvinsz]

fixes #3434

The fix is basically to listen for the StateChanged event of the parent Window.
If the window is restored from the minimized state, we focus the previously focused element, which got stored in a field _lastFocusedDialogElement when the window got minimized.

I want to point out the artificial delay I added, that we have in other places in the DialogHost class too. I just couldn't get it to work without the (50ms) delay, as described in my comment above the "hacky" code.

...
// Kinda hacky, but without a delay the focus doesn't always get set correctly because the Focus() method fires too early
Task.Delay(50).ContinueWith(_ => this.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() =>
...

[corvinsz]

It seems like there is a problem with running the tests via github actions. This PR just had another failing test (SplitButton_WithButtonInPopup_CanBeInvoked). When reran, the test passed.
I'm not sure if this is a TUnit or github actions problem?

I had the same issue in #3915