Use C++20 in RemoteVstPlugin

[messmerd]

Reland C++20 mode compilation for RemoteVstPlugin

• Fix a weird issue with std::wstring on wineg++ by using std::unique_ptr<wchar_t[]> instead. See my comment here. This fixed the RemoteVstPlugin crash in C++20 mode.
• Minor RemoteVstPlugin cleanup
  • Fix a few memory leaks by using std::unique_ptr or stack-allocated objects
  • Add some missing pluginDispatch constants
  • Use std::int16_t instead of short
  • Better VstSyncData member names
  • Add a few TODO comments
• Rename F_OPEN_UTF8 to fopenUtf8 since it's not a macro anymore
• Use C++20 in our determine_version_from_source CMake function
• Update the ZynAddSubFX submodule since it (unfortunately) depended on LMMS's IoHelper.h header to build

Known issues:

• 64-bit OTT fails to load

[sakertooth]

Great, this seems to have fixed the crash in #7624, and the code changes look fine to me. Seems ready for merge 👍

[messmerd]

@sakertooth Thanks! I noticed an issue when trying to open 64-bit OTT though, and it didn't seem to be happening on master, so I'd like to investigate that first

[sakertooth]

@sakertooth Thanks! I noticed an issue when trying to open 64-bit OTT though, and it didn't seem to be happening on master, so I'd like to investigate that first

Did a quick test run and also noticed a strange deadlock that I usually don't get in the waitForMessage function. Not sure if this PR was in any way related though, but its a possibility.