ProjectExplorer: Introduce RunWorker::reportDone()

To be used for short-lived tasks to signal that they are done.
Essentially an optional reportStarted() followed by reportStopped()

Using it in GdbServerPortsGatherer when the port list is ready fixes
the wrong "un-stopped" situation in RemoteLinux debugging when
the inferior finishes by itself.

Change-Id: I310831a7875fe6f2c598302b73cda6c9669efb1d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/debugger/debuggerruncontrol.cpp

@@ -685,7 +685,7 @@ void GdbServerPortsGatherer::handlePortListReady()
             return;
         }
     }
-    reportStarted();
+    reportDone();
 }
 
 // GdbServerRunner