Simplify file transfer dialog UI
    
    When transferring a large number of files, the file transfer dialog was
    unusable because the window size would be larger than the client
    desktop. To solve this, remove the list of individual files (and the
    ability to cancel each file transfer independantly) and only display
    a single overall progress bar that shows the status of all ongoing
    transfers.
    
    This also allows us to remove the delayed unref of the task since we
    don't need to show the task information about each individual transfer
    task until the window is closed. Removes TaskFinishedData type.
    
    This patch requires new API from spice-gtk to calculate the overall
    progress:
     spice_file_transfer_task_get_total_bytes()
     spice_file_transfer_task_get_transferred_bytes()