AttachUnknownControl doesn't work with wxBitmapButton derived classes

[Christoph Borgolte]

I try to insert my own control wich derives from wxBitmapButton to an 
xml-generated panel.
This works fine under wxGTK but doesn't under wxMSW.

The problem is: The attached control is not visible, even thogh the 
events are send when you push the mousebutton at the place where the 
control should reside.
Using wxPanel derived classes works.

Any ideas?