gattrib: Include footprints that are defined in symbols

[KaiMartin]

Let gattrib parse the symbols definition for footprint
attributes. At the moment it shows only the footprint
values that are set in the schematic. That way gattrib
doesn't help me to find the symbols that have no
footprint at all.

Yes, I know, that this touches the dreaded light/heavy
symbols debate. However, I as a user like to set the
footprint on a per symbol basis. Especially as some
symbols will have their specialized footprint with no
chance that they will ever be associated with a
different footprint.

---<(kaimartin)>---