Fix layout cycle error in ProportionalStackPanel

[BAndysc]

As reported in #366, resizing panels led to "Layout Cycle Detected" warning. This is not caused by a cycle like the message suggest, but we were changing Proportion from the MeasureOverride and ArrangeOverride methods.

https://github.com/wieslawsoltes/Dock/blob/master/src/Avalonia.Controls.ProportionalStackPanel/ProportionalStackPanel.cs#L224

I don't know if there is a better place to call AssignProportions (outside Measure and Arrange methods). Thus my proposal is to avoid calling InvalidateMeasure/InvalidateArrange when AssignProportions is being executed. AffectsParentMeasure is just a syntax sugar for

PROPERTY.Changed.AddClassHandler<TElement>((sender, e) =>
{
    if (sender.GetVisualParent() is not TParent parent)
        return;

    parent.InvalidateMeasure();
});

So this PR basically only adds a flag to skip invalidating in this case.

Fixes: #366