ValidationAssist.PopupPlacement does not work as expected

I have the following in my Xaml:
```xaml

```
I expect the popup to show up on the right of the TextBox. But instead I get the following:


Results are the same with MaterialDesignInXaml v3.1.2 and v3.2.0-ci1479.

There is another issue too. Alignment by the top edge produces bad looking result. VerticalAlignment is required for side placement, and better default to Center.

Start by reproducing the reported XAML TextBox case with ValidationAssist.PopupPlacement="Right" and ValidationAssist.UsePopup="True", then inspect the implementation of those properties. Done means the validation popup appears beside the TextBox for side placement and the top-edge alignment produces the expected result.

Written by the indexing model from the issue text.