added support for percentage values
e.g. heightDock="x:#Cover.70" would be 70% of the width of element #Cover
The code above does not work, for some reason.
Also "align" attribute for drawing squares does not work properly when used along with percentage values.
With fixed values, it's working fine. For instance, with the following code:
<?xml version="1.0" encoding="utf-8"?>
<root>
<element name="Cover(1)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" width="200" height="200" align="Bottom.Right" min="50" max="500" brightness="1.0" />
<element name="Cover(2)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" width="200" height="200" align="Bottom.Left" min="50" max="500" brightness="1.0" />
<element name="Cover(3)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" width="200" height="200" align="Top.Right" min="50" max="500" brightness="1.0" />
<element name="Cover(4)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" width="200" height="200" align="Top.Left" min="50" max="500" brightness="1.0" />
</root>
You get this:

But how should you code to get the following scalable squares, each side of which is as long as a quarter of panel height?

I tried the following code, but it shows no covers:
<?xml version="1.0" encoding="utf-8"?>
<root>
<element name="Cover(1)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" widthDock="y:Panel.25" width="0" heightDock="y:Panel.25" height="0" align="Bottom.Right" min="50" max="500" brightness="1.0" />
<element name="Cover(2)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" widthDock="y:Panel.25" width="0" heightDock="y:Panel.25" height="0" align="Bottom.Left" min="50" max="500" brightness="1.0" />
<element name="Cover(3)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" widthDock="y:Panel.25" width="0" heightDock="y:Panel.25" height="0" align="Top.Right" min="50" max="500" brightness="1.0" />
<element name="Cover(4)" type="AlbumCover" xAnchor="Panel.Center" x="0" yAnchor="Panel.Center" y="0" widthDock="y:Panel.25" width="0" heightDock="y:Panel.25" height="0" align="Top.Left" min="50" max="500" brightness="1.0" />
</root>