Fix plugin rename and replace-with against latest changes

Closes #479

source/backend/engine/CarlaEngine.cpp

@@ -723,6 +723,7 @@ bool CarlaEngine::addPlugin(const BinaryType btype, const PluginType ptype,
             plugin->setVolume(oldVolume, true, true);
 
         plugin->setActive(wasActive, true, true);
+        plugin->setEnabled(true);
 
         callback(ENGINE_CALLBACK_RELOAD_ALL, id, 0, 0, 0.0f, nullptr);
     }

source/backend/engine/CarlaEngineJack.cpp

@@ -1271,6 +1271,7 @@ public:
             // reload plugin to recreate its ports
             plugin->reload();
             plugin->loadStateSave(*saveStatePtr);
+            plugin->setEnabled(true);
         }
 
         return plugin->getName();

source/carla_host.py

@@ -658,10 +658,10 @@ class HostWindow(QMainWindow):
 
         if not forced:
             ask = QMessageBox.question(self, self.tr("Warning"), self.tr("There are still some plugins loaded, you need to remove them to stop the engine.\n"
-                                                                        "Do you want to do this now?"),
-                                                                        QMessageBox.Yes | QMessageBox.No, QMessageBox.No)
+                                                                         "Do you want to do this now?"),
+                                                                         QMessageBox.Yes | QMessageBox.No, QMessageBox.No)
             if ask != QMessageBox.Yes:
-                return
+                return False
 
         return self.slot_engineStopTryAgain()