Frontend/jobs: When stopping a scheduler, don't hesitate to stop the active job. Otherwise quitting Paperwork may take a very long time

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Remove 0.5s wait time before generating thumbnails

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Drop the wait time on the imgbuilder as well

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Also remove the wait time on the BoxRefresher (not the box selector !)

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Jobs: stop correctly jobs that have already been started once

Signed-off-by: Jerome Flesch <jflesch@wyplay.com>
Merge branch 'testing' into unstable