Gerbview: Add support for recent File Format Attribute metadata, both for X2 Gerber file format and X1 (R274X) format.

Pcbnew already uses this attribute when creating Gerber files.
Because Gerber files using this attribute identify the board layers stackup, Gerbview (if this attribute is defined) can sort gerber images stach up like the board.
(in layer manager, just right click to access to the sort menu)

gerbview/dialogs/dialog_print_using_printer.cpp

@@ -37,6 +37,7 @@
 
 #include <gerbview.h>
 #include <gerbview_frame.h>
+#include <class_GERBER.h>
 #include <pcbplot.h>
 
 static double s_ScaleList[] =
@@ -163,7 +164,7 @@ void DIALOG_PRINT_USING_PRINTER::InitValues( )
         msg << wxT( " " ) << ii + 1;
         m_BoxSelectLayer[ii] = new wxCheckBox( this, -1, msg );
 
-        if( g_GERBER_List[ii] == NULL )     // Nothing loaded on this draw layer
+        if( g_GERBER_List.GetGbrImage( ii ) == NULL )     // Nothing loaded on this draw layer
             m_BoxSelectLayer[ii]->Enable( false );
 
         if( ii < 16 )