Display spinner for locked folders

[PVince81]

When a folder is locked by high-level file locking (#16237) whenever a transaction is in progress (ex: move), show a spinner on it in the web UI.

It might be possible to detect locked folder when retrieving the list of folders and adding specific attributes in the response.

@icewind1991 @jancborchardt

(prerequisite is that #16237 is merged and finished)

[PVince81]

See the steps from #15994 that illustrates how it could be triggered.

[PVince81]

I just realized that it cannot work, because while we can display a spinner when a folder is locked, we cannot remove the spinner because there is no way to get notified whenever a folder has just become unlocked.

Well, unless we have some kind of push notification service, but that feels like overkill.

[PVince81]

Not a good idea, see previous comment