Simplify the CMake file even more

- target_sources(... PUBLIC ...) doesn't do what I thought it did, and has no useful purpose here.
- Experimental features are an Alpha only thing, and Unstable features should not be part of a Candidate release.
- ENABLE_LTO is not a flag anymore, as CMake has a global flag for it.

While we haven't split out Core from the main file yet, and we still keep running into strange duplicate symbol or undefined symbol errors, this will hopefully simplify the CMake file further. End goal is to eventually split StreamFX into smaller sub-plugins that can operate mostly independently. At some point, the goal is to be able to soft-depend on other components, i.e. Blur can softly depend on Dynamic Mask, and then have extra features if the component is installed. This is not quite fleshed out yet, and I have no clear idea on how to make it work.