Pcbnew: minor Geda and KiCad plugin improvements.

* Make GPCB_PLUGIN::EnumerateFootprints() read the directory contents
  instead of loading the entire cache.
* Make KICAD_PLUGIN::EnumerateFootprints() read the directory contents
  instead of loading the entire cache.
Make old behavior the default in EnumerateFootprint() changes.
Pcbnew: minor Geda and KiCad plugin improvements.

* Make GPCB_PLUGIN::EnumerateFootprints() read the directory contents
  instead of loading the entire cache.
* Make KICAD_PLUGIN::EnumerateFootprints() read the directory contents
  instead of loading the entire cache.