Polishing of NotificationManager:

Fixed pairing of PrintObjects with slicing warning notifications.
Removed or commented out dead code.
Added documentation.

src/libslic3r/SLAPrint.hpp

@@ -420,6 +420,8 @@ public:
 
     void                clear() override;
     bool                empty() const override { return m_objects.empty(); }
+    // List of existing PrintObject IDs, to remove notifications for non-existent IDs.
+    std::vector<ObjectID> print_object_ids() const;
     ApplyStatus         apply(const Model &model, DynamicPrintConfig config) override;
     void                set_task(const TaskParams &params) override;
     void                process() override;