This adds Mach thread priority support (both in the application and realtime band) and recalculates thread priorities when the process priority changes.
Part 3, which is still a bit WIP deals with implementing priority boosts (for main threads and threads which are processing window messages), effectively fully replacing https://gitlab.winehq.org/wine/wine/-/merge_requests/1232.
Currently the implementation in this MR already technically overrides what https://gitlab.winehq.org/wine/wine/-/merge_requests/1232 does, if it makes sense I can also revert it here.
I added a few comments regarding the Mach thread priority API usage, as there is limited documentation available, and much was inferred from the source or by testing. If this is too verbose I can also remove that...