Fix minor bug in eeschema (opening a relative path does not work) (patch from HennerZeller).

* make some more fields 'const' that can.
* Instead of previous/next _visible_ element, Go through
      previous and next element. Otherwise the cursor stops moving
      if the item is only partially visible.

Fix a minor error in class D_PAD: void D_PAD::Flip( int Y ) changed to virtual void D_PAD::Flip( const wxPoint& aCentre ) (as defined in BOARD_ITEM)
Scripting: fix compatibility current pcbnew version in 2 examples and the default extension of board files  in board.i (was .kicad_brd, now is .kicad_pcb)

gr_basic.cpp: rewrite the function which draws the outlines of a thick segment.

* 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

  Factored out the next-pad-number function

Use environment KISYS3DMOD to know the default path for 3D shapes in footprint editors dialogs, and build short 3D file names.
This value is now shown in these dialogs.

If it is not defined by user, it is set to a legacy 3D files path.
The default value needs to be fixed on MACOSX.
This change could be better, but it is compatible with environment variables and the old way ( defined search paths) to find 3D shape files.
3D shapes files names can  also include an environment variable definition, like libraries names.
However using KYSYS3DMOD in these names is not required.
If the 3D filename is a legacy name, KYSYS3DMOD is automatically used as default path.

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