Rework on 3D viewer (work in progress): Disable or enable layers, grid or 3D footprint shapes should be now faster.

Eeschema:  * Preselect the currently chosen component in the browser when pressing the thumbnail view.
* Various smallish documentation updates in the component chooser area.

* Allow to select units in components that have more than one right in the
  component chooser dialog.
* Keep chosen unit in history.
* Show preview of current component unit as thumbnail image next to the
  description box.
* Fixes lp:1280567

* Apply Henner Zeller's patch to add connecting line from reference and labels to component position whilst moving to help identify which component the reference or label belongs too

Add a workaround to fix a new bug in wxWidgets 3.0 ( Windows specific ) which shows icons only with a size = 16x16 in menus which have attribute wxITEM_CHECK

Fix some incorrect icons (which were using a not supported font for texts). Add ps_router icon from Konstantin Baranovskiy.

Moved netnames (GAL specific layers) to a separate enum, to avoid saving/reading their settings from files.
Added a check for the number of PCB_VISIBLE elements.
Worksheet & general purpose overlay layers are visible by default.

Changed BOARD_DESIGN_SETTINGS::m_VisibleElements from int to long, to assure at least 32 bits length (without depending on the platfrom int size).

Add missing calls to Show( false ) to some main frames (Kicad, eeschema, gerbview), to force config saving when closing these windows.
Code cleanup and minor coding style fixes  in footprint_wizard_frame

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

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

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.

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