Improve restore method on data fingerprint changes

Currently, the client will switch its sync algorithm to restore mode when detecting a change of the data fingerprint (webdav property on root folder).
That switch will be sticky until a full sync is completed and the new value is stored on client side.
The restore mode includes those changes:
* deleted files/folders on the server but synced on client side will be uploaded again
* files/folders on client side will be considered more recent even if their metadata disagree

The downside of this is that a client may be stuck in restore mode breaking the overall working of shared folders/files. That would happen if there are unrelated sync errors preventing a full synchronization to be completed.

The idea is to replace this with a one time notification presenting a list of files or folders to restore and asking for confirmation to the user. That way the client could still preserve some data missing from a backup while preventing continuous disruption.
It is also important to find a way to let users cooperate on shared folders to prevent conflicting attempts to restore those shared files.