Replace submodules with CPM

See website docs preview for description.

After merge you may want to cleanup removed submodule folders in ThirdParty:

git clean -ffd ThirdParty

Speeds up CMake runs (every CPM dependency cached) by 2-3 seconds because of fewer git-calls from inside CMake.

Moved RapidXML and nlohmann/json into CPM dep as well.

Also removed (unused) VtkFbxConverter functionality.

---

Future work could try to replace Conan entirely. A first step would be to replace Boost and VTK packages with CPM. This would allow the standard cli config to be built without any outside package manager or system installed packages.

---

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?