Added code to support a cache option for filebrowser widget.

This new option will allow code that uses the filebrowser to specify
whether or not to cache files read from the server when clicking on
a path.  If not using cache, files will be re-read from the server.

Change-Id: I30cd04211a712855f2404fabcc4d8a8854e5f06f
Merge topic 'pvweb-add-filebrowser-cache-option' into master

847d7503 Added code to support a cache option for filebrowser widget.