Add Initial Qt6 Build Support

[simpsont-oci]

Problem:

• Build files are not currently set up to build for Qt6
• A few changes in Qt6 APIs break the build for Qt6

Solution:

• Expand build coverage to include Qt6
• Fix build for Qt6 APIs

[simpsont-oci]

At some point, we should update the CI/CD workflows to actually verify this continues to build with Qt6 (it does (and runs) on my machine, for what it's worth). We can convert this to draft if you want me to look into that eventually. Or we can open an issue and someone can do it later.