Provide graphical details when uploading a new big file

When a new local file will be sent to the server, and it it is quite big, the app will show the syncing icon in the systray.
But it may be seen as ineffective because the upload will not start before having computed the checksum. And doing so for a 100 GB file will take a while.

An idea would be to use the same progress bar we are using to check downloads integrity. Like that, the user can follow the checksum computation with a nice progress bar and one now knows that the application is doing something.

The intesresting code that take a while to finish is here: https://github.com/nuxeo/nuxeo-drive/blob/release-4.4.3/nxdrive/engine/processor.py#L886-L888