Added KiCad-style modification methods (hover over an item and press a hot key, without selecting first).
Modification point is selected basing on the number of selected items.
Rotation angle setting (Preferences->General) is taken into account while rotating.

- pcbnew.InteractiveSelection.Single for selecting a single item
- pcbnew.InteractiveSelection.Clear for clearing the selection
Made SELECTION_TOOL::clearSelection private.

SELECTION_TOOL::SELECTION::Clear made private, as there was no chance to call it outside the SELECTION_TOOL class.

Moved ClearSimple() functions back to the header file.

Another way of handling items for the ratsnest (clearer and now finally supports undo/redo of the PNS created tracks).

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.