Dual-mode garbage collection support

A lot of progress was made towards this in [153]. It might actually just work now, since I think I've taken care of all the weak links. I've yet to test it.

Okay, so it works with the Test App if the Test App is made garbage collected-only. But please test this with your projects and let me know how things go.

What I mailed to Andy, for the record:

I've been testing Sparkle GC with Aurora now, and monitored mem usage,
etc. - It's looking pretty good so far, I haven't seen any anomalies
and memory usage is as expected.

I gotta admit I'm not using GC in Aurora production releases though. I
was planning to do so until a few weeks before 4.0, but I decided I
had non-GC code (retain, release) anyway and the GC overhead was thus
not necessary. People writing GC apps have probably been compiling
Sparkle for GC anyway (as I did back then, and it worked okay - beta
testers were running it, ...), so I'd say it's pretty safe.

Alright, I've got three "works for me"s, so I'm going to say it's done. :D