Add name to internal threads and make it possible to set properties to them

On systems with limited resources there is often a need to finetune it, by setting the thread priorities and stack size of internal threads. Qt 5.15 for VxWorks had such implementation, but there is a need to have it on other OSes too.

Naming the internal threads would help with the debugging and with the communication with the users, because it would be visible if the problematic thread is internal or not.