Fix AVM1 event broadcaster to correctly pass on arguments to listeners

Using `Array.prototype.slice.call(arguments, 1)` didn't work at all, but `...args` does.
Change AVM1 MovieClipLoader to pass target MovieClip to listeners instead of the MovieClipLoader itself
Dispatch loader events after all frame events in the event loop

Because Flash does that, too. To pull this off, we have to also delay committing frames from the parser until the right point of the event loop. For everything but the main SWF's first frame, that is: committing that is what triggers any code execution whatsoever. ... Which we might want to change at some point, too.
Dispatch PROGRESS events every event loop turn if actual loading progress was made

 ... instead of only once when loading starts and then again when loading is complete. This regressed in Spring, without anyone noticing. Which isn't all that surprising: it's close to impossible to build a good, deterministic testcase for this.
Improve as3 Loader reftest to also test which frame events are dispatched in

The previous two patches fix parts of our Loader event dispatching behavior in ways that weren't tested by this reftest before.