Change UI job's process() method to be protected, as it should have been

Also add some comments to the interface of Job class

src/slic3r/GUI/Jobs/ArrangeJob.hpp

@@ -35,7 +35,9 @@ protected:
     void prepare() override;
 
     void on_exception(const std::exception_ptr &) override;
-    
+
+    void process() override;
+
 public:
     ArrangeJob(std::shared_ptr<ProgressIndicator> pri, Plater *plater)
         : PlaterJob{std::move(pri), plater}
@@ -46,8 +48,6 @@ public:
         return int(m_selected.size() + m_unprintable.size());
     }
     
-    void process() override;
-    
     void finalize() override;
 };