Add SpacingProperty to VirtualizingStackPanel

[Neakita]

What does the pull request do?

This PR adds SpacingProperty from StackPanel to VirtualizingStackPanel and complements MeasureOverride and ArrangeOverride accordingly

What is the current behavior?

At the moment, VirtualizingStackPanel does not have SpacingProperty and instead you have to use Margin on items and negative Margin in their common container to achieve the same result.

What is the updated/expected behavior with this PR?

Port SpacingProperty from StackPanel to VirtualizingStackPanel taking virtualization into account.

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

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

Fixed issues

Fixes #17336

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052800-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[Neakita]

@cla-avalonia agree

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052854-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052858-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052868-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Neakita]

Seems to be working now:

ControlCatalog.Desktop_KiXoMxkd3k.mp4

although there are some TODO’s that require attention:
https://github.com/Neakita/Avalonia/blob/fd2d0f4bb201c9e9034b5f87d6e7f47bc4a6d9bc/src/Avalonia.Controls/VirtualizingStackPanel.cs#L233
https://github.com/Neakita/Avalonia/blob/fd2d0f4bb201c9e9034b5f87d6e7f47bc4a6d9bc/src/Avalonia.Controls/VirtualizingStackPanel.cs#L719

and also the obvious lack of tests (they are only two and they do not check, actually, virtualization), about which I need some third party opinion.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052870-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]