Avalonia: Add support for setting Pen properties from Xaml

[wieslawsoltes]

Repro:

<GeometryDrawing x:Key="Bulb" Brush="#75FF0000" Geometry="M0 0L60 0L60 60L0 60L0 0Z">
    <GeometryDrawing.Pen>
        <Pen Brush="#FFFF0000" Thickness="8" LineCap="Round" LineJoin="Round" MiterLimit="10"/>
    </GeometryDrawing.Pen>
</GeometryDrawing>

Error:

Error       Unable to find suitable setter or adder for property Brush of type Avalonia.Visuals:Avalonia.Media.Pen for argument System.Private.CoreLib:System.String, available setter parameter lists are:
 (line 10 position 30) Line 10, position 30.    RenderDemo  C:\DOWNLOADS\GitHub-Forks\Avalonia\samples\RenderDemo\Pages\DrawingPage.xaml    10  

[grokys]

Ah yes, Pen is currently an immutable object. We need to make it mutable.