In order to implement the backup and restore of log stores, that
are created as subplugins of the tool_log plugin , we need to
extend subplugins support from activities to virtually any plugin.
Basically that implies moving the add_subplugin_structure() method from
its current, restricted, activity level to general backup_structure_step.
This commit implements the change in backup, covered with tests verifying
old, bc behavior and also new, general one.
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.
