Use wxDC for all coordinate manipulations.

* Remove all occurrences if #ifdef USE_WX_ZOOM and all associated code within
  the #else/#endif block ( old zoom code ).
* Removed the build option for USE_WX_ZOOM from CMakeList.txt and config.h.in.
* Removed all scaling code in base screen object.
* Fixed buffered paint and buffered client DC on Windows.  Buffering works
  properly on Linux and Windows.
* Modified kicad_device_context.h to automatically uses buffering on platforms
  where double buffering is supported natively.
* Remove all of the scaled versions of the drawing functions in gr_basic.cpp
  and any support code.
* Removed all traces of ActiveScreen global variable from eeschema and
  gerbview.
* Renamed Recadre_Trace to RedrawScreen in draw frame object.
* Renamed PostDirtyRect to RefreshDrawingRect in draw panel object.
* Lots of code cleaning an Doxygen comment improvements.

common/base_struct.cpp

@@ -399,11 +399,7 @@ void EDA_TextStruct::DrawOneLineOfText( EDA_DRAW_PANEL* aPanel, wxDC* aDC,
     if( aAnchor_color != UNSPECIFIED_COLOR )
     {
 
-#if USE_WX_ZOOM
         int anchor_size = aDC->DeviceToLogicalXRel( 2 );
-#else
-        int anchor_size = aPanel->GetScreen()->Unscale( 2 );
-#endif
 
         aAnchor_color = (EDA_Colors) ( aAnchor_color & MASKCOLOR );