Strongly-typed inline styles

[h0lg]

This PR enables adding inline IStyle instances to any StyledElement, allowing you to write styles in a strongly-typed manner without .xaml files - even down to the selectors.

let private coloredTextBoxWatermark (color: IBrush) =
    let style = Style(_.OfType<TextBox>().Template().OfType<TextBlock>().Name("PART_Watermark"))
    style.Setters.Add(Setter(Avalonia.Controls.TextBlock.ForegroundProperty, box color))
    style

let view () =
    AutoCompleteBox([]).inlineStyles(coloredTextBoxWatermark(Brushes.Crimson))

It includes

• an example that uses the new extension to style an AutoCompleteBox in a way that neither the Fabulous nor Avalonia API would otherwise allow for.
• an unrelated fix for the focusing of AutoCompleteBoxes in the EditableTreeView example.

[edgarfgp]

Thanks for this. Really appreciate all your contributions.