Undo buffer for the PNS router. Still, it has a bug - it crashes when an undo/redo operation is performed while routing a track.

RN_DATA::GetNets() -> RN_DATA::GetNet() with an assert to check if someone calls it for the unconnected net.
Only items that belong to a net are removed from ratsnest.

BOARD::chainMarkedSegments() uses a safer method for gettings pads.

Fixed crashes of ratsnest when a pointer for an item has changed after undo/redo operations.
Vias are properly removed from ratsnest (pcbnew/class_board.cpp).

RN_DATA::Update()/AddSimple() take BOARD_ITEM* as the parameter (instead of being split to versions with BOARD_CONNECTED_ITEM* and MODULE*), to make the code look clearer.

Added BOARD_[CONNECTED_]ITEM::IsConnected() for checking if a BOARD_ITEM is BOARD_CONNECTED_ITEM as well.

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

Add EDA_BASE_FRAME::Show() and call SaveSettings() only from there for all derived wxFrames.  Remove calls to SaveSettings() elsewhere.

github_plugin.cpp: in error message relative to"cannot get  zip archive...", line 496, make the sentence relative to http GET command not translatable
inside the full translated message.

Remove g_LibraryNames and all supporting infrastructure from pcbnew.  FP_LIB_TABLE is now local to major wxFrames.

Pcbnew: fix bug #1272714 . Code cleanup: use 2 different enums for pad shapes and pad drill shapes. Remove never used pad shape.

Increase boost download TIMEOUT to 10 minutes, offer a LOG_DOWNLOAD option for capturing download problems.

Fix not working + and - keys in layers manager.
Fix typo errors and modify some messages to make translator's life more easy.

NETINFO_ITEMs are not stored in a vector anymore, instead they are held in a unordered_map. Now, the net codes may be not consecutive. There is another way for assigning net codes (using a static int that holds a possible empty net code and a function that makes sure it is not used [getFreeNetCode()]).
Removed some unused fields (NETINFO_ITEM::m_NbNodes, m_NbLink, m_NbNoconn, m_Flag).

BOARD_CONNECTED_ITEMs do not store net code anymore (m_NetCode field), instead net info is stored using a pointer to NETINFO_ITEM.
GetNet() refers to the net code stored in the NETINFO_ITEM. SetNet() finds an appropriate NETINFO_ITEM and uses it.
Removing GetNet() & SetNet() (and the whole net code idea) requires too many changes in the code (~250 references to the mentioned functions).
BOARD_CONNECTED_ITEMs by default get a pointer to NETINFO_ITEM that stores unconnected items. This requires for all BOARD_CONNECTED_ITEMs to have a parent (so BOARD* is accessible). The only orphaned item is BOARD_DESIGN_SETTINGS::m_Pad_Master, but it does not cause any issues so far.
Items that do not have access to a BOARD (do not have set parents) and therefore cannot get net assigned, by default get const static NETINFO_LIST::ORPHANED.

Performed tests:
- loaded .kicad_pcb, KiCad legacy board, Eagle 6.0 board, P-CAD board - all ok
- load a simple project, reload netlist after changing connections in eeschema - ok
- save & reload a board - ok, but still contain empty nets
- remove everything, restore with undo - ok
- remove everything, reload netlist - ok
- changing net names (all possibilites: empty->existing, empty->not existing, existing->empty, existing->not existing) - all ok
- zones: when net is changed to a net that does not have any nodes besides the zone itself, it does not get filled