- ZONE_CONTAINER::m_Netname field
- ZONE_CONTAINER::SetNetName()
- ZONE_CONTAINER::SetNet() [it uses the one in BOARD_CONNECTED_ITEM]
- ZONE_CONTAINER::GetNetName() [instead BOARD_CONNECTED_ITEM::GetNetname is used]
- ZONE_CONTAINER::SetNetNameFromNetCode()

Performed tests:
- Drawn a zone that belongs to a net, then modified schematics so the net does not exist anymore. After reloading the net list, all pads/tracks are updated. Zones still belongs to the net that does not exist in the schematic (but still exists in .kicad_pcb file). After running DRC, the zone becomes not filled.
- Undo & redo affects assignment of a polygon to a specific net (you may change net of a polygon, refill it and undo/redo the changes).