ShowInTaskBar with Owned window: make consistent with WPF

[danwalmsley]

Repro

var window = CreateSampleWindow();

window.ShowInTaskbar = false;

window.Show(GetWindow());

window.Loaded += (sender, args) => { window.ShowInTaskbar = true; };

Current behavior (windows only)
When the loaded event happens, it will just remove the owner window. Meaning that the dialog will go behind the main window.

I checked WPFs behavior and it definately ignores ShowInTaskBar changes in this scenario.

1. ShowInTaskBar = false, causes SetParent to be called and parent set to HiddenWindow.Handle, and _hiddenWindowIsParent = true;

2. Show gets called, SetParent sets the new owner. all good.

3. ShowInTaskbar gets set to true, but because _hiddenWindowIsParent was left true, it overwrites the owner window.
   casuing the issue.

Fixed behavior:

ensure _hiddenWindowIsParent is always in sync.
ShowInTaskbar is ignored and relationship maintained.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038580-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]