Scheduler now has the ability to run jobs on multiple repositories concurrently and run multiple jobs on one repo (by queuing them).
For each repository, there is one queue. I have represented a queue by a 'site'. Between sites (ie repository), tasks run concurrently. On one site, tasks run one by one. The user also run tasks by adding them to the queue but he can't run multiple backups because start backup button is disabled when a job is running.
* Simplify non-blocking BorgThread.run.
* Fix issue with displaying nested folders in extract-dialog.
* Fix error text expansion.
* Add many new tests. Dont open main window on startup.
