Get rid of the Avalonia tool window

[ForNeVeR]

There's an Avalonia tool window that's used to host the logs from the previewer process. I don't think it's of use for ordinary users. Let's drop it, and show the stderr/stdout in the IDE log instead.

[rstm-sf]

This is a good idea, otherwise windows are created for each xaml and it is not clear which one it belongs to.