DataGrid - preserve groups collapsed state

[ltetak]

What does the pull request do?

DataGrid has a Grouping behavior. It has a major flaw: when the collection is Reset or just when the DataGrid control is removed and re-attached to the visual tree, it forgets the collapsed state of groups.

What is the current behavior?

Open the Control catalog DataGrid page.
On the Grouping tab collapse some groups.
Switch tabs back and forth.
Groups are expanded again.

What is the updated/expected behavior with this PR?

The state is saved and restored

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

I tried to mimick the selection state cache on the same spot.

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

none

Obsoletions / Deprecations

Fixed issues

[MrJul]

Thank you for your contribution.

However, the team recently decided to move DataGrid to its own repository. As part of this move, we won't accept new features for this control.

Please see this discussion for details.

However, this PR looks like a useful bug fix.
Could you please re-open the PR on the new repo?