1) Moved first_layer_heigth frrom PrintObjectConfig to PrintConfig.

Thus the first_layer_height is no more object specific. That makes
   a lot of sense due to the brim calculation being performed over
   all layers at once and due to future merging of supports of
   different objects at first layer.
2) Because now first_layer_height is print specific, the relative
   first layer height derived from the object layer height was partially
   disabled: First the relative first layer height is converted to
   an absolute value when importing config, second the side text
   was changed from "mm or %" to "mm". Still the UI allows entering %.

Both changes may be controversial, let's wait for user feedback.

src/libslic3r/GCode.cpp

@@ -1111,7 +1111,7 @@ void GCode::_do_export(Print& print, FILE* file, ThumbnailsGeneratorCallback thu
     // Write some terse information on the slicing parameters.
     const PrintObject *first_object         = print.objects().front();
     const double       layer_height         = first_object->config().layer_height.value;
-    const double       first_layer_height   = first_object->config().first_layer_height.get_abs_value(layer_height);
+    const double       first_layer_height   = print.config().first_layer_height.get_abs_value(layer_height);
     for (const PrintRegion* region : print.regions()) {
         _write_format(file, "; external perimeters extrusion width = %.2fmm\n", region->flow(*first_object, frExternalPerimeter, layer_height).width());
         _write_format(file, "; perimeters extrusion width = %.2fmm\n",          region->flow(*first_object, frPerimeter,         layer_height).width());