Added: DXF Multilayer plotting

DXF layers and appropriate layer colors added.  Option mimics the PDF
multilayer option

Fixes https://gitlab.com/kicad/code/kicad/-/issues/2526

pcbnew/pcb_plot_params.cpp

@@ -116,6 +116,8 @@ PCB_PLOT_PARAMS::PCB_PLOT_PARAMS()
     m_colors                     = m_default_colors.get();
 
     m_blackAndWhite              = true;
+
+    m_DXFExportAsMultiLayeredFile = false;
 }
 
 
@@ -340,6 +342,9 @@ bool PCB_PLOT_PARAMS::IsSameAs( const PCB_PLOT_PARAMS &aPcbPlotParams ) const
     if( !m_outputDirectory.IsSameAs( aPcbPlotParams.m_outputDirectory ) )
         return false;
 
+    if( m_DXFExportAsMultiLayeredFile != aPcbPlotParams.m_DXFExportAsMultiLayeredFile )
+        return false;
+
     return true;
 }