Make the progress bar smooth during file-transfer
    
    When the transfer of a file finishes we stop considering that file's
    size in the progress bar which makes it move back due the new
    'transfer size' and 'transferred bytes' - for all the other files.
    
    This patch aims to keep the progress smooth when a file is finished
    using the notify::total-bytes from SpiceFileTransferTask to be aware
    of all file's sizes.
    
    Note that as we have only one progress bar for all files being
    transferred, it is expected that it will go back when a new
    file-transfer operation starts (e.g we drag-and-drop new files while
    we are already transferring other files).
    
    As requested, this patch also updates the string message to include the
    amount of files that will be transferred in case we have more than one
    file.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1449572
    Signed-off-by: Victor Toso <victortoso@redhat.com>
    Acked-by: Pavel Grunt <pgrunt@redhat.com>