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/FillBedJob.hpp

@@ -24,6 +24,7 @@ class FillBedJob : public PlaterJob
 protected:
 
     void prepare() override;
+    void process() override;
 
 public:
     FillBedJob(std::shared_ptr<ProgressIndicator> pri, Plater *plater)
@@ -35,8 +36,6 @@ public:
         return m_status_range;
     }
 
-    void process() override;
-
     void finalize() override;
 };