Improve sequencer/scheduler timing + ordering

[adroitwhiz]

Right now, there doesn't seem to be much effort put into matching the timing and order of script execution with Scratch. This may be the cause of a large number of compatibility issues.

We should go through the thread-scheduling logic here and compare it with Scratch's (or my refactoring of it) and try to make them match. We can likely reuse many scratch-vm execution tests.