don't show folders that do not contain subfolders or audio files

hopefuly this will fix crash on RasPi

LenaPi/controllers/NavigationController.h

@@ -22,6 +22,7 @@ public:
 private:
     void setContextProperties();
     void add(const QString & path, NavigationItemModel* parentItem);
+    bool checkContent(const QString& path);
 
     NavigationItemModel* mRootItem;
     NavigationListModel* mNavList;