Gerbview: remove dependencies from Pcbnew.

Should allow Pcbnew code easier to change and Gerbview code more understandable and easier to maintain.
Code cleaning (remove dead code, add comments).
Minor other enhancements.

gerbview/dcode.cpp

@@ -286,11 +286,9 @@ void GERBVIEW_FRAME::CopyDCodesSizeToItems()
 {
     static D_CODE dummy( 999 );   //Used if D_CODE not found in list
 
-    BOARD_ITEM*   item = GetBoard()->m_Drawings;
-
-    for( ; item; item = item->Next() )
+    GERBER_DRAW_ITEM* gerb_item = GetItemsList();
+    for( ; gerb_item; gerb_item = gerb_item->Next() )
     {
-        GERBER_DRAW_ITEM* gerb_item = (GERBER_DRAW_ITEM*) item;
         D_CODE*           dcode     = gerb_item->GetDcodeDescr();
         wxASSERT( dcode );
         if( dcode == NULL )