CMake: Restructuring ideas

Since project inception, CMake was taken and modified to fit from cmake template by Jason Turner (lefticus on github) But for multiplatform use, another project has appeared which has this worked out better Iris engine it's made without submodules as we use them, but instead uses CMake FetchContent Parts that are platform dependent, are in separate folders with target_sources conditional adding of subdirectories, and many more tricks - including working CTest setup. Worth considering to restructure our build system to use something similar to what iris is using.