Move Zyn GUI to main thread

[tresf]

Closes #703

We'll need some solid testing on Windows and Linux to make sure no regressions are caused as a result of this change.

[PhysSong]

As the author of this patch, I'd like to say that it should be fine on every platforms. The reasons are:

1. It is safe to execute the message loop in any thread since it doesn't call UI functions or thread-dependent functions(as far as I know).
2. It is normal to run UI loop in main thread. It's safe, too.
3. I've changed the synchronization logic a little bit. Now the message processing thread always finishes before the GUI loop. So there should'n be any threading issues.

Anyway, testing is always good. 👍

[tresf]

Here's a build for testing this out: https://github.com/tresf/lmms/releases/download/v1.2.0-rc4/lmms-1.2.0-rc5.3-mac10.13.dmg

[tresf]

We've extensively tested this on Mac, Linux and Windows.

Here's some evidence that the Zyn UI can remain open and stable with this PR despite several preset changes. Worth noting... I was able to get it to crash twice, but I cannot reproduce now despite how hard I try. When it crashed, the stack trace did not point to the UX, so @PhysSong suspects I had found an existing edge case that already exists.