feat: add TreeViewItemIndentConverter for dynamic margin calculation

[zdpcdt]

What does the pull request do?

This pull request refactors how indentation is applied to TreeViewItem elements in both the Fluent and Simple themes of Avalonia.

What is the current behavior?

What is the updated/expected behavior with this PR?

It introduces a new converter, TreeViewItemIndentConverter, to handle indentation dynamically, replacing the previous static converters and hardcoded implementations.

How was the solution implemented (if it's not obvious)?

Introduction of a new converter:

• src/Avalonia.Controls/Converters/TreeViewItemIndentConverter.cs: Added a new TreeViewItemIndentConverter class implementing IMultiValueConverter. It calculates the indentation based on the Level and Indent values, returning a Thickness object.

Updates to the Fluent theme:

• src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml: Replaced the TreeViewItemLeftMarginConverter with the new TreeViewItemIndentConverter. Updated the Grid.Margin property in the PART_Header element to use a MultiBinding with the new converter. [1] [2]

Updates to the Simple theme:

• src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml: Removed the LeftMarginConverter and replaced it with a TreeViewItemIndent resource. Updated the Grid.Margin property in the PART_Header element to use a MultiBinding with the new TreeViewItemIndentConverter. [1] [2]

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

#18311

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056162-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[zdpcdt]

@cla-avalonia agree

[MrJul]

API diff for review:

+namespace Avalonia.Controls.Converters
+{
+    public class TreeViewItemIndentConverter : IMultiValueConverter
+    {
+        public static readonly TreeViewItemIndentConverter Instance;
+        public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture);
+    }
+}

[rabbitism]

API diff for review:

+namespace Avalonia.Controls.Converters
+{
+    public class TreeViewItemIndentConverter : IMultiValueConverter
+    {
+        public static readonly TreeViewItemIndentConverter Instance;
+        public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture);
+    }
+}

This can be internal.

[MrJul]

Notes from the API review:

We spent quite a bit of time debating whether this should be added to Avalonia.Controls.Converters, be internal, or be public but theme-specific.

First, we rejected internal to allow users to easily copy and paste an existing ControlTheme to their own project, without having to copy a bunch of related internal types.

Regarding whether the converter should be part of Avalonia.Controls or Avalonia.Themes.<...>, we believe consistency is the most important thing here. There are already several theme-specific converters such as MenuScrollingVisibilityConverter or MarginMultiplierConverter in the Avalonia.Controls.Converters namespace (Avalonia.Controls assembly).

We might consider moving those converters to theme-specific assemblies in the future. If we do so, it will be for all converters.

As such, this API is accepted as-is.

[MrJul]

LGTM, thank you!