custom panel with xaml+codebehind in designer

I am writing a custom panel should be used in xaml like this:

<custom:TackPanel Orientation="Horizontal">
<custom:TackPanel.Trackdefinitions>
<custom:TrackDefinition Size="10" x:Name="Track1" />
<custom:TrackDefinition Size="*" x:Name="Track2"/>
<custom:TrackDefinition Size="2*" x:Name="Track3"/>
</custom:TackPanel.Trackdefinitions>
<Button TrackPanel.Track="0" />something<Button>
<Button TrackPanel.Track="1" />somethingelse<Button>
</custom:TrackPanel>

My problem now ist that the custom panel is internally composed out of
several other panels and controls (grid, stackpanel, scrollviewer).
This composition should ideally be done in a xaml file (like the vs
extension does for a UserControl). The codebeind.cs then works with
these internal controls depending on what TrackDefinitions and
childelements get added.

How can I (preferably using the vs extension) create a custom panel
that has a xaml file attached to a codebehind .cs file ?