Add ground pour and multilayer support

[ceremcem]

Steps for ground pour:

• Add polygon support. Polygon must be editable
• Subtract all geometry from this polygon (paperjs-offset)
• Adding new geometries must update this polygon.
• Add support to connect this polygon to any netid. This should "remove" any trace with this netid in that layer and connect that trace to the polygon.
• Polygon tool must support adding more nodes and moving existing nodes.

Steps for multilayer support:

• Footprints should only be sent to B.Cu and F.Cu layers. Additional layers may only support traces, ground pours and vias.
• Vias may not connect all layers.