Improve code quality - Githubissues

Currently my goal is to improve the code quality overall. I'm not so proud of some areas of the codebase as I once may have been. 😤

Some goals

[x] Splitting videoplayer window from it's underlying implementation (mpv)
[x] rework serialization/deserialization of configuration files and project files
[x] remove OFS_ThreadPool/OFS_AsyncIO
[ ] reduce use of god object OpenFunscripter::ptr global where possible
[x] rework keybinding system to enable new features
[x] remove mirror mode (it doesn't serve it's inteded purpose)
[ ] split OpenFunscripter into atleast two files separating window related code from application related code
[x] OpenFunscripter::ActiveFunscriptIdx should be managed by OFS_Project
[x] consider adding and using eventpp instead of SDL2s event system
[x] ~rewrite the 3D simulator~

OpenFunscripter / OFS