Streamline code, avoid functions and container.end() invocations,
introduce private, reduce #ifdefs, clarify code and add
comments about usage on various platforms.
Implement QFileIconProvider interface, making it usable for file system
models. Use in FolderNavigationWidget. On this occasion, add checkable
"Synchronize" action to FolderNavigationWidget.
Task-number: QTCREATORBUG-62