Added NETINFO_MAPPING, to ease saving nets with consecutive net codes (without modifying the net codes during the run time).
Now, nets are saved with consecutive net codes (both modern & legacy plugins).
Zones are saved together with their nets, without depending on the fact if there are any pads with such net. Therefore validation of zone net names was removed (pcbnew/class_board.cpp).

Performed tests:
- Changed a pad's net name from empty to existent - ok, name was changed.
- Changed a pad's net name from empty to nonexistent - ok, error message is displayed, net name stays empty.
- Changed a pad's net name from existent to empty - ok, net name became empty
- Changed a pad's net name from existent to nonexistent - ok, error message is displayed, net name is not changed.

- 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).

- KiCad s-expr & legacy, Eagle, P-CAD boards seem to load without any problem (they also contain correct net names assigned to the appropriate pads). All types of board file formats were loaded, then saved in sexpr format and reopened with a KiCad built from the master branch (without my modifications).
- A few boards were also saved using the legacy format and were opened with the master KiCad without any issues.

- Change a net name for a pad, restore with undo/redo - ok
- Remove everything, restore with undo - ok
- Remove everything, reload netlist - ok

Differences observed between files saved by the master branch KiCad and this one:
- list of nets are not saved in any particular order, so net codes may differ
- the default net class does not contain the unconnected net