[gta02-core] fped: mounting holes and through-hole pads

[Werner Almesberger]

fped now supports holes. Holes are either for through-hole pads/pins
(when they're on a copper pad) or for mounting holes (when they're
not).

In the case of through-hole pins, fped also propagates the surface
layers (copper, paste, mask) to the bottom side.

Holes are constructed in the same way that rounded pads are made, by
defining the enclosing rectangle. In fped's toolbar, the hole tool
is below the pad tools.

We don't have any through-hole components in gta02-core, but we have
a few mounting holes that couldn't be properly modeled so far, e.g.,
in the USB connector.

- Werner