General backup/restore does not need to handle external data sources natively - so
any changes needed to achieve this should be contained to the plugin that needs it.
When there are many files inside a single resource, it's possible
for the backup to time out because it does not report progress
while adding entries to the backup_ids table.
Adds calls to the new backup progress tracking API within various steps of the
backup system - previously it only tracked progress between steps, but some steps
can themselves be slow. This ensures the system displays progress (either by
moving the progress bar if possible, or by making the wibbler below it pulsate)
during nearly all of the backup process.
