Fixes crashes on quitting

[poke1024]

Fixes the crashes described in #76, which are due to:

• the audio thread calling into V8 after it's been already destroyed
• the V8 Global destructor running in the application shutdown after exit(), which crashes on macOS

EDIT Tested on d14d0d6 (currently packaged KodeStudio version), since master doesn't work at all on macOS currently due to Metal issues. Works fine now on d14d0d6 though.

[RobDangerous]

Oh, sorry, too much changed for this one but we should have a look a closing behavior again soon.

[poke1024]

I'll see if I can find time to look at the new code and if is has any of the effects.

For the record, there were two separate problems:

(1) the core audio thread under macOS wasn't stopped on quit, i.e. a call to Kore::Audio2::shutdown(); was missing, and the thread crashed when accessing stuff that the main thread had already destroyed
(2) various V8 refs, that were not reset before quit, caused crashes in their shared ptr destructors (I don't remember details).

Krom switched from V8 to Chakra? Cool.

[RobDangerous]

Aye. And debugging is finally working...