Fix reftrack init and reference restriction

Because a treeitem is created early in the reftrack constructor,
the reftrack must at least have all attributes set. Add restricted attribute
to self before creating the treeitem.

Restrict reference when status is Imported.
Fix reftrack reference restriction

The fix before was wrong. You can only reference if the status is None.
Hide reference button if restricted, Frames to Widgets

Frames in reftrack widget have borders. Widgets do not. That looks
cleaner.
Make delete restrictable