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()