MBS-7596: The relationship editor dialog should work on a copy of the relationship until the dialog is accepted

Creates a clone of the relationship to-be-edited, then copies the changes over in the dialog's accept method. This makes editing things faster but makes accepting the dialog a bit slower, since it has to apply everything at once (whereas previously everything was already applied, and it just hid the dialog, basically). But I'm investigating performance improvements outside of this patch.