Unicode. Rename UTF8toUTF16 to U8toU16 and switch back to std::wstring
instead of std::u16string.

- Add Engine::fromJson which can be called without the GUI at all.
- Make RackWidget::fromJson attach itself to existing Engine state. However, once attached, they own their Engine objects (Module, Cable).
- Remove Engine thread. Now Engine must be stepped by other threads, such as the audio thread. This is fantastic because there is no longer a mutex lock every audio buffer.
	- Add concept of the "primary module", which is allowed to call Engine::step().
- Add many Module events.