Hackfix for calling SelectAll with SelectedItem binding

[grokys]

What does the pull request do?

As described in #13676, calling SelectAll() on a ListBox with no current selection and a two-way SelectedIndex or SelectedItem binding in place, causes SelectAll() to fail. What was happening is:

1. SelectAll() is called
2. This selects all items, and updates the SelectedIndex/SelectedItem property on the SelectionModel
3. This causes PropertyChanged to be raised for SelectedIndex/SelectedItem on the ListBox
4. The binding system writes the new value to the source
5. The binding system notices that the value has changed in the source
6. The binding system writes the new value back to the target SelectedIndex/SelectedItem, even though this value hasn't changed
7. Setting SelectedIndex/SelectedItem's semantics are such that setting them clears all other selected items

Step 6 is a bug in the binding system: we should not be writing an unchanged value back to the target due to a change in the source, and this will be fixed in the upcoming binding system refactor. However in the meantime, I think we need a hackfix as an interim solution. The easiest way to do this is to add the fix to SelectionModel.

Fixed issues

Fixes #13676

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0042725-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0042771-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]