* Add tests for binding negation operator.
Expected results come from 11.0.x branch.
* Unset and null need to be distinct.
Also remove `ExpressionNode.Reset` as it was doing unneeded stuff and should just be the same as `SetSource(AvaloniaProperty.UnsetValue, null);`.
* Make ExpressionNode.OnSourceChanged accept null.
The previous design assumed that a source of `null` was an invalid input to all types of expression nodes. It turned out that there was a single exception to this rule: the `!` operator can in fact operate on a null value. With this new design we instead have to explicitly check for a null value in every override of `OnSourceChanged ` except in `LogicalNotNode`.
Due to this change, we also have to distinguish between `null` and `(unset)` in `ExpressionNode.SetSource` as well.
Fixes#16071
* Fix comment.
And implement the functionality in the content controls themselves. `ContentControlMixin` was too complex and even with its complexity had bugs (such as in #2821). By moving the functionality to the content controls there is some repeated code but it's much more straightforward.
Steven Kirk
Dariusz Komosinski
Nikita Tsukanov
Steven Kirk
Andrey Kunchev