Initialize the PCB_LAYER_T enum in the property manager

Since all the property descriptions are static initialization, each instance
should contain the code to initially populate the enum if it is empty,
otherwise an assert is thrown warning the enum may be empty.

pcbnew/class_module.cpp

@@ -1679,6 +1679,16 @@ static struct MODULE_DESC
 {
     MODULE_DESC()
     {
+        ENUM_MAP<PCB_LAYER_ID>& layerEnum = ENUM_MAP<PCB_LAYER_ID>::Instance();
+
+        if( layerEnum.Choices().GetCount() == 0 )
+        {
+            layerEnum.Undefined( UNDEFINED_LAYER );
+
+            for( LSEQ seq = LSET::AllLayersMask().Seq(); seq; ++seq )
+                layerEnum.Map( *seq, LSET::Name( *seq ) );
+        }
+
         wxPGChoices fpLayers;       // footprints might be placed only on F.Cu & B.Cu
         fpLayers.Add( LSET::Name( F_Cu ), F_Cu );
         fpLayers.Add( LSET::Name( B_Cu ), B_Cu );

pcbnew/class_track.cpp

@@ -1107,6 +1107,16 @@ static struct TRACK_VIA_DESC
             .Map( VIATYPE::BLIND_BURIED, _( "Blind/Buried" ) )
             .Map( VIATYPE::MICROVIA, _( "Microvia" ) );
 
+        ENUM_MAP<PCB_LAYER_ID>& layerEnum = ENUM_MAP<PCB_LAYER_ID>::Instance();
+
+        if( layerEnum.Choices().GetCount() == 0 )
+        {
+            layerEnum.Undefined( UNDEFINED_LAYER );
+
+            for( LSEQ seq = LSET::AllLayersMask().Seq(); seq; ++seq )
+                layerEnum.Map( *seq, LSET::Name( *seq ) );
+        }
+
         PROPERTY_MANAGER& propMgr = PROPERTY_MANAGER::Instance();
 
         // Track