Make all Loader methods native

... because the remaining AS3-implemented ones didn't do anything useful, anymore.
Clean up file loading and remove parser thread

The interactions between Loader, ResourceLoader, and Parser were a jungle of epic proportions. This patches turns epic into substantial.
Retire parsing worker

With the previous patch, it's not used anymore.
Add SwfTag name to warning about dropped tags

Reinstated after having been lost to a merge conflict earlier.
Reduce iteration count in Stage unit test `displayListStateConsistency`

Shaves off 1s of unit tests runtime. Which is almost 30% of total time for all tests.