Progress bar updater: Let it run in an independant thread, otherwise it's useless

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Always make sure to stop the worker thread

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Multi-scan: Fix

Signed-off-by: Jerome Flesch <jflesch@gmail.com>