Add geben-backtrace-refresh, to refresh the backtrace window without selecting it
Automatically refresh the stack trace buffer if it is visible
Optionally restore frame configuration at the end of a session

If geben-save-frame-configuration is called at the beginning of a session, the
current frame configuration is restored at the end of the session.
fixup! Add geben-backtrace-refresh, to refresh the backtrace window without selecting it
Prepare to optionally use a fixed layout for debugger windows

When requesting to display a buffer, specify the requested role, and use a
matching window if present.

Actually, the role could be an attribute of the buffer, but this suffices for now.
Enable fixed layout if frame configuration was saved during setup

In theory one could want to save the frame configuration without enabling fixed
layout, but I'm ignoring this for now.