Implemented the LAYER_NUM typedef (LAYER was already taken as a class name...) to represent a layer number.

Pcbnew: more about keepout areas: remove keepout for pads option and add keepout for copper zones option.
Export keepout to Specctra DSN file.

This is *a work in progress*, so some features are missing, and/or could be modified.
Mainly keepout zones are not yet exported to autorouters, and pads are not taken in account.
Some code cleanup in polygon.*