Tracking RMC4_UE5 changes vs master

[connorjak]

This might not be merged for a long time, or ever, but it's useful to have a PR open for the reviewing tools.

Per @Moddingear: there are macro definition issues which block having a single codebase that works for both UE4 and UE5.

We should still do our best to retain compatibility in terms of making it possible to port non-UE5-specific changes back to master branch.