1) Add 32 Cu Layers.

2) Change from legacy Cu stack to counting down from top=(F_Cu or 0).
   The old Cu stack required knowing the count of Cu layers to make
   sense of the layer number when converting to many exported file types.
   The new Cu stack is more commonly used, although ours still gives
   B_Cu a fixed number.
3) Introduce class LSET and enum LAYER_ID.
4) Change *.kicad_pcb file format version to 4 from 3.
5) Change fixed names Inner1_Cu-Inner14_Cu to In1_Cu-In30_Cu and their
   meanings are typically flipped.
6) Moved the #define LAYER_N_* stuff into legacy_plugin.cpp where they
   can die a quiet death, and switch to enum LAYER_ID symbols throughout.
7) Removed the LEGACY_PLUGIN::Save() and FootprintSave() functions.
   You will need to convert to the format immediately, *.kicad_pcb and
   *.kicad_mod (=pretty) since legacy format was never going to know
   about 32 Cu layers and additional technical layers and the reversed Cu
   stack.

pcbnew/pcbplot.cpp

@@ -44,67 +44,42 @@
 #include <macros.h>
 
 
-wxString GetGerberExtension( LAYER_NUM layer )
+const wxString GetGerberExtension( LAYER_NUM aLayer )
 {
-    switch( layer )
+    if( IsCopperLayer( aLayer ) )
     {
-    case LAYER_N_FRONT:
-        return wxString( wxT( "gtl" ) );
+        if( aLayer == F_Cu )
+            return wxT( "gtl" );
+        else if( aLayer == B_Cu )
+            return wxT( "gbl" );
+        else
+        {
+            return wxT( "gbr" );
+        }
+    }
+    else
+    {
+        switch( aLayer )
+        {
+        case B_Adhes:       return wxT( "gba" );
+        case F_Adhes:       return wxT( "gta" );
 
-    case LAYER_N_2:
-    case LAYER_N_3:
-    case LAYER_N_4:
-    case LAYER_N_5:
-    case LAYER_N_6:
-    case LAYER_N_7:
-    case LAYER_N_8:
-    case LAYER_N_9:
-    case LAYER_N_10:
-    case LAYER_N_11:
-    case LAYER_N_12:
-    case LAYER_N_13:
-    case LAYER_N_14:
-    case LAYER_N_15:
+        case B_Paste:       return wxT( "gbp" );
+        case F_Paste:       return wxT( "gtp" );
 
-        // TODO: see if we use .gbr or a layer identifier (gb1 .. gbnn ?)
-        // according to the new internal layers designation
-        // (1 is the first internal layer from the front layer)
-        return wxString( wxT( "gbr" ) );
+        case B_SilkS:       return wxT( "gbo" );
+        case F_SilkS:       return wxT( "gto" );
 
-    case LAYER_N_BACK:
-        return wxString( wxT( "gbl" ) );
+        case B_Mask:        return wxT( "gbs" );
+        case F_Mask:        return wxT( "gts" );
 
-    case ADHESIVE_N_BACK:
-        return wxString( wxT( "gba" ) );
-
-    case ADHESIVE_N_FRONT:
-        return wxString( wxT( "gta" ) );
-
-    case SOLDERPASTE_N_BACK:
-        return wxString( wxT( "gbp" ) );
-
-    case SOLDERPASTE_N_FRONT:
-        return wxString( wxT( "gtp" ) );
-
-    case SILKSCREEN_N_BACK:
-        return wxString( wxT( "gbo" ) );
-
-    case SILKSCREEN_N_FRONT:
-        return wxString( wxT( "gto" ) );
-
-    case SOLDERMASK_N_BACK:
-        return wxString( wxT( "gbs" ) );
-
-    case SOLDERMASK_N_FRONT:
-        return wxString( wxT( "gts" ) );
-
-    case DRAW_N:
-    case COMMENT_N:
-    case ECO1_N:
-    case ECO2_N:
-    case EDGE_N:
-    default:
-        return wxString( wxT( "gbr" ) );
+        case Dwgs_User:
+        case Cmts_User:
+        case Eco1_User:
+        case Eco2_User:
+        case Edge_Cuts:
+        default:            return wxT( "gbr" );
+        }
     }
 }