InspectorPlugin: Handle disconnect

Disonnect inspector when qml adapter emits disconnected
signal.

Change-Id: I2f9acbe2cc19c44edf500e7329d1590b73f97113
Reviewed-by: Christiaan Janssen <christiaan.janssen@nokia.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>

src/plugins/qmljsinspector/qmljsinspector.cpp

@@ -349,6 +349,7 @@ void InspectorUi::disconnected()
     m_clientProxy = 0;
     m_propertyInspector->clear();
     m_pendingPreviewDocumentNames.clear();
+    setDebuggerEngine(0);
 }
 
 void InspectorUi::objectTreeReady()