Klaus,
Maybe this is a fault in the IDE or more likely the user! To start from scratch, I closed the Designer, removed the library reference from the IDE and closed the IDE.
Then
Open the IDE, add a tick on AnotherdatePicker library. Close the IDE. (just to be sure it saved the lib references). Opened the IDE, opened my Project, opened the designer ... No sign of the datepicker under AddView->Customview
Any thoughts?