Instruments: Have the Zynaddsubfx UI embed like VST(i) plugins

### Enhancement Summary

You can see a setting for embedding VST(i) plugins into the LMMS editor itself, so you can use it without it spawning a new window. can't we do that with zynaddsubfx?

### Implementation Details / Mockup

Perhaps using the QT embedding thing or win32, i dont know.

A mock up

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by locating LMMS's existing VST(i) UI embedding path and the ZynAddSubFX UI integration. Compare the Qt and Win32 approaches mentioned in the issue, then determine what cross-platform window handling is required. Done means the ZynAddSubFX UI can be used inside the LMMS editor instead of opening a separate window.

Written by the indexing model from the issue text.