draft: upgrade codebase to qt6 and cmake

[RTUnreal]

This is a very rough first try of an upgrade of the codebase to qt6. ATM, it contains the following:

• [x] automatic conversions from easylog++ to qt6's builtin logger (still not missing the configurable log filename)
• [x] a basic cmake generator
  • [x] test on windows (msvc 2022 buildtools)
  • [x] test on linux (NixOS)
  • [ ] further clean up automatically generated CMakeLists.txt file
  • [x] replace resource references in CMakeLists.txt with the AUTORCC feature in cmake
• [ ] CPP fixes
  • [x] upgrade src/overlaycontroller.cpp to use Qt's new rendering pipeline (use this as inspiration)
  • [ ] make graphics api selection dependant on a setting (using QQuickWindow::setGraphicsApi; see this)
  • [ ] replace vr:VROverlay()->SetOverlayFromFile with something qrc compatible
  • src/tabcontrollers/AudioTabController.cpp
  • src/tabcontrollers/RotationTabController.cpp
  • src/tabcontrollers/UtilitiesTabController.cpp
  • src/tabcontrollers/VideoTabController.cpp
• [ ] upgrade packaging
  • [ ] replace homebrew packaging with cpack (nsis,etc.)
  • [ ] move src/res/img/icons/ into seperate directory
• [ ] other cleanups
  • [ ] decide ~what to do with unused assets~ if we want to switch to vector graphics by QSvgRenderer
  • src/res/img/audio/microphone/ptt_notification.svg
  • src/res/img/rotation/noautoturn.svg

Bugs:

• [ ] Settings are all enabled, for whatever reason, when started

This list is not exhaustive and might change/grow later.