qmljs: add isIdle method to detect background parsing/scannig

If the qmljsmodel has no background task, then isIdle returns true

Change-Id: I31420343c560c5ed118f371f7be347eadb6622f4
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/libs/qmljs/qmljsmodelmanagerinterface.h

@@ -182,6 +182,7 @@ public:
     void emitDocumentChangedOnDisk(QmlJS::Document::Ptr doc);
     void updateQrcFile(const QString &path);
     ProjectInfo projectInfoForPath(QString path) const;
+    bool isIdle() const ;
 
     PathsAndLanguages importPaths() const;
     QmlJS::QmlLanguageBundles activeBundles() const;