pcb_backannotate should be implemented with scheme api

[Dan McMahill]

The pcb_backannotate script is a perl script which loads an annotation file produced by PCB and applies the changes to a set of schematic file(s). Rather than implementing its own .sch file parser, this utility should be written using the gaf scheme api. That would take advantage of the parser which is already implemented and also encourage new and similar utilities to take advantage of the api instead of adding more and more tools which implement their own parsers and are tightly married to today's file format.

The same comment applies to 'pads_backannotate' and 'refdes_renum'.