gEDA-user: pcb: active layer helpers

[DJ Delorie <dj@xxxxxxxxxxx>]

Ok, two features I just checked in that I'd like some feedback on.

1. If you're looking at the component side of the board, and the
   component layer is the active layer, and the solder layer is
   hidden, and you flip the board over... the solder layer is shown
   and made active and the component layer is hidden.  Same when you
   flip back.

   I.e. if you set it up so that it's like a real board, pcb will
   attempt to preserve the illusion.

2. When multiple layers are visible, the non-active layer groups are
   drawn in a slightly faded color.