Fixed memory leaks in ContextMenu.cs

[adirh3]

What does the pull request do?

Fixes two memory leaks -

1. Leak when controls are not freed when they have any ContextMenu set.
2. Leak when ItemPanel is set to StackPanel;' I don't know why.

What is the current behavior?

Two issues:

1. When you have a Control with ContextMenu applied, for example from a ControlTheme, there will be only 1 ContextMenu created for all controls of these type, but each time a control is created it will be added to _attachedControls and will stay there forever.
2. For some reason, when a ContextMenu with ItemsSource binding have an ItemsPanel as StackPanel the items created will not be cleared. When changing ItemsPanel to VirtualizingStackPanel there is no issue.

What is the updated/expected behavior with this PR?

No memory leaks when using ContextMenu

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

1. Add and remove attached controls when they attach and detach from visual tree.
2. Replace override of default ItemsPanel, so it uses VirtualizingStackPanel like every other place. I saw that this line was from 2019, and I can't think of any particular reason why it needs to stay.

Note - the issue when using StackPanel is not resolved, but I didn't have time to investigate it.

Checklist

• Added unit tests (if possible)? I added but can't test it, all DotMemory unit tests fail for some reason
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038437-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]