When synchronizing the front end with the back end after Undo / Redo

jump, postpone error messages, so they are displayed after
the Undo / Redo jump has been fully performed.
Otherwise there would be a message box opening, taking over the message
queue, and possibly performing actions as rendering on an inconsistent
application state.

src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.hpp

@@ -87,7 +87,7 @@ public:
     bool is_in_editing_mode() const { return m_editing_mode; }
     bool is_selection_rectangle_dragging() const { return m_selection_rectangle.is_dragging(); }
     bool has_backend_supports() const;
-    void reslice_SLA_supports() const;
+    void reslice_SLA_supports(bool postpone_error_messages = false) const;
 
 private:
     bool on_init();