Add a spinner when opening a folder in the gui

[touilleMan]

Opening a folder can take some time if the corresponding folder manifest is not in the local cache (hence a request must be send to the backend).
Under a slow connection it provides a weird feeling given the double click on the folder name doesn't do anything for a couple of seconds (given the impression the user has done a missclick), it would be better to display a spinner next to the name of the folder being opened.