DockingTabControl doesn't work when within a UserControl

[Kryptos-FR]

Description

Sometimes it is useful to split a big UI into smaller UserControls. However, it doesn't currently work with this library because the DockingHost expects SplitPanels and DockingTabControls to be direct children of one another (

Avalonia.UpDock/Avalonia.UpDock/Controls/DockingHost.cs

Lines 101 to 118 in d749ec0

	private void HandleChildrenModified(Control control, NotifyCollectionChangedEventArgs e)
	{
	foreach (var child in e.OldItems?.OfType<Control>() ?? [])
	{
	if (child is SplitPanel splitPanel)
	UnregisterSplitPanel(splitPanel);
	else if (child is TabControl tabControl)
	UnregisterTabControl(tabControl);
	}
	foreach (var child in e.NewItems?.OfType<Control>() ?? [])
	{
	if (child is SplitPanel splitPanel)
	RegisterSplitPanelForDocking(splitPanel);
	else if (child is TabControl tabControl)
	RegisterTabControlForDocking(tabControl);
	}
	}

).

While it is possible that doing a deep iteration of all children might be expensive and/or trigger unexpected behaviors, I do think that UserControl should be special cased and its children looked up.

Example expected to work

Adapting the sample from the README.

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:up="using:Avalonia.UpDock.Controls"
        xmlns:view="using:MyProject"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="CODE_BEHIND_CLASS_GOES_HERE"
        Title="Example">

    <up:DockingHost>
        <up:SplitPanel Fractions="1, 1" Orientation="Horizontal">
            <up:DockingTabControl>
                <TabItem Header="Tab A">
                    <TextBlock Margin="5">Content</TextBlock>
                </TabItem>
            </up:DockingTabControl>
            <view:MyView />
        </up:SplitPanel>
    </up:DockingHost>
</Window>

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:up="using:Avalonia.UpDock.Controls"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="MyProject.MyView">
    <up:DockingTabControl>
        <TabItem Header="Tab B">
            <TextBlock Margin="5">More content</TextBlock>
        </TabItem>
        <TabItem Header="Tab C">
            <TextBlock Margin="5">Even more content</TextBlock>
        </TabItem>
    </up:DockingTabControl>
</UserControl>