Drive: allow to change home folder location once initialized

Since NXP-12657 the Nuxeo Drive home folder can be configured in the Settings popup when first launching Drive, yet it can not be updated later on.

We should allow this as Dropbox does, by letting the user chose another location then displaying a message like:
"This operation will move the Nuxeo Drive folder and all its content from its current location to '<new_location>'."
Note that in this case we will need to wait for the synchronization to be finished before moving the files to the new location.