Upgrade KF5 version for 5.2.0 release.
Save processing order in each step.

This preserves the order of the entries in the columns.
Add missing licenses. Simplify SortRole.
Simplify progress reporting.
Implement image loader job class.
Add ComputeThumbNailJob.
Show reasonable detail information about the thumbnails.
Disable open files action while processing.
First steps at an example average load manager.

Only supports Unix OSes for now.
Use a QImageReader to be able to detect error causes.
Implement a naive version of the average load manager.

The problem is, it creates a situation of memory starvation.
Fix WeaverLazyThreadCreationTest, limit the number of created threads.
Sebas spotted a typo.
Allow setting the worker count to zero in the demo.

This is new, until now the queue always required the worker count to be
larger than zero. There is no inherent reason for that in ThreadWeaver,
and there may be applications that make use of it.
When load management is enabled, disable manual controls for workers.