Modernize project to conform to standards in LSL and XDF ecosystem

[cboulay]

I'm slowly trying to modernize all active projects in the LSL and XDF ecosystem. By modernizing, I mean the following:

• CI: GHA runners to build and test for Win x64, Mac Universal, Linux x64
• CD: GHA runners to create installer packages for Win x64, Mac Universal, Linux x64
• CD: Code-signing and notarizing Mac Universal content and installer
• For GUI apps (like this one): Qt5 -> Qt6
• For some GUI apps: Provide a CLI-only option (not relevant here)
• Automatic versioning from git tags created during release
• Automatic attachment of installers to GitHub release