Move the Design tab's Bed toggle into the Feature tree, under the Origin row

The bed's show/hide state is now remembered across sessions like the Origin
row's, shown by default. The GUI ladder's ribbon x-coordinates are shifted by
the removed checkbox's derived width and still need re-measuring on the rig.

src/libslic3r/AppConfig.cpp

@@ -367,6 +367,10 @@ void AppConfig::set_defaults()
     // restores the conventional CAD representation for users who expect it (x0kd).
     if (get("design_connector_face_glyph").empty())
         set_bool("design_connector_face_glyph", true);
+
+    // Design tab: draw the printer bed and its plate grid (the Feature tree's Bed row).
+    if (get("design_show_bed").empty())
+        set_bool("design_show_bed", true);
 #endif
 
 //#ifdef SUPPORT_SHOW_HINTS