Switched PBRemoteProgressSheet to PBTask

[ksuther]

I gave this branch a try and switched PBRemoteProgressSheet to use PBTask while I was trying it out.

It also seems like the readDataToEndOfFile calls in PBTask are unnecessary since the readabilityHandler will read the data. They were throwing exceptions for me when a PBTask was run.

[tiennou]

I'm really sorry. That last commit is "unwanted", because it's likely to conflict with my fix/sheets branch, which is responsible for gutting the whole concept of having a "tasked" progress sheet (see 778a7428ec30e0a3c7b19cfbcd1d7b83e3619f0e for the gory details). I'll check to see if anything can be salvaged though.

I'd gladly integrate the other two in my "branch mess" though, so ❤️ for that.

If you want to help still, here are a few pointers :

• feature/pbtask is the new PBTask implementation
• feature/task-wrangling is about switching GitX to use PBTask exclusively
• wip/repo-splitting is about moving PBGitRepository out of the document hierarchy
• fix/sheets is about simplifying the "messy" memory behavior we have w.r.t to sheets

So 1) should be complete (modulo bugs), 2) is "in-progress", 3) is in need of testing/moving around/fixing whatever is broken with document-handling (things like the age-old window restoration bug, the recent document controller that nobody ever sees, etc.), 4) should be complete and just needs some testing.

Again, I'm really sorry for not having that "upstream", but it's all a bunch of pretty large changes, and I'm not keen on having a broken master (even though it's generally quite usable, because at some time I was running on top of that fix/sheets branch without any evident issues).