[libcalamares] Restore finish() slot to job queue

FIXES #1507

src/libcalamares/JobQueue.cpp

@@ -135,6 +135,7 @@ public:
         {
             emitProgress( 1.0 );
         }
+        m_runningJobs->clear();
         QMetaObject::invokeMethod( m_queue, "finish", Qt::QueuedConnection );
     }
 

src/libcalamares/JobQueue.h

@@ -77,9 +77,15 @@ signals:
      */
     void queueChanged( const QStringList& jobNames );
 
-private:
+public slots:
+    /** @brief Implementation detail
+     *
+     * This is a private implementation detail for the job thread,
+     * which should not be called by other core.
+     */
     void finish();
 
+private:
     static JobQueue* s_instance;
 
     JobThread* m_thread;