corrections in work description

++pcbnew & gerbview

  * added g_CurrentTrackList, and made g_CurrentTrackSegment and g_FirstTrackSegment

    be #defines into this DLIST<TRACK>, see pcbnew.h.

  * Switched datatype of BOARD::m_Track, BOARD::m_Zone, etc to DLIST<>.

  * Changed to DLIST<> for almost all the list heads in BOARD.

    Switched datatype of BOARD::m_Track, BOARD::m_Zone, etc to DLIST<>.

    This simplifies BOARD::~BOARD() somewhat, and adds automatic counting of list elements and encapsulation

    of all list operations into DHEAD member functions, see common/dlist.cpp.

  * Removed TRACK::Insert() since it no long fits with the new BOARD::m_Track and BOARD::m_Zone datatypes.

    the only way to put a TRACK on a list is through DLIST<TRACK>::PushFront()

    and other member functions of DLIST<TRACK>.

  * Switched to std::vector<TRACK*> in track.cpp to hold the temporary static list.

  * Made g_UnDeleteStack be a DLIST<TRACK>.

  * renamed some KICAD_T to more closely match the corresponding classname.

  * Changed to DLIST<> for almost all the list heads in BOARD.

  * Renamed some KICAD_Ts to more closely match the corresponding classname.

++gerbview

  * switched to GR_COPY mode for drawing so we can see the erasures.  debugged

  * Switched to GR_COPY mode for drawing so we can see the erasures.  debugged

    erasures in rs274d.cpp.

  * Now using order specific TRACK loading, so erasures come after objects they

    are intended to erase.  Still need layer ordering and polygon ordering support, my todo.