[dev] How to add a button to a newly created toolbar?

[Matthias Benkmann]

I've managed to create my own toolbar with the following code

layout = thisComponent.CurrentController.Frame.LayoutManager
layout.createElement( "private:resource/toolbar/UITest" )
layout.showElement( "private:resource/toolbar/UITest" )

but I can't seem to find a way to add a button to it. Could someone please 
give me a pointer in the right direction. Thanks.

Matthias