QML Inspector: Don't update debug id's when not debugging

Creating LookupContext seems to be expensive, so don't update anything
unless debug mode is already running.

src/plugins/qmljsinspector/qmljsinspector.cpp

@@ -523,7 +523,8 @@ ProjectExplorer::Project *InspectorUi::debugProject() const
 
 bool InspectorUi::isShadowBuildProject() const
 {
-    if (!debugProject())
+    // for .qmlproject based stuff, build dir is empty
+    if (!debugProject() || debugProjectBuildDirectory().isEmpty())
         return false;
 
     return (debugProject()->projectDirectory() != debugProjectBuildDirectory());