Hollowing task triggered by the gizmo now spawns a UI job to not block the UI thread

The AABB tree calculation is still done in the UI thread, so it gets blocked for some time

src/slic3r/GUI/Plater.hpp

@@ -190,6 +190,7 @@ public:
     void reload_from_disk();
     bool has_toolpaths_to_export() const;
     void export_toolpaths_to_obj() const;
+    void hollow();
     void reslice();
     void reslice_SLA_supports(const ModelObject &object, bool postpone_error_messages = false);
     void changed_object(int obj_idx);