* new option `upload_achievements_icons_to_gpu` in `configs.main.ini` which controls whether the overlay should upload the achievements icons to the GPU and display them or not
* synchronize overlay proc with the periodic steam callback in a better way to avoid FPS drop
* prevent overlay flickering regression
* upload achievements icons to the GPU in the overlay proc periodically, this dramatically decreased the startup locking/halt time
* fix a potential deadlock scenario in the overlay as a result of synchonizing with 2 mutex objects
* convert the animation duration to millisec once during settings parsing
* decrease the notification margin from 10.0 to 5.0, it looked way shifted on larger screens
* pass around instances on std::chrono instead of float, use float in the final stage when processing/operating-on the time (if needed)
* decrease the animation duration to 0.35 sec, looks more swishy!
* added a new button to the overlay `"Test achievement"` which triggeres a test achievement
* added a new overlay appearance option `Achievement_Unlock_Datetime_Format` which allows changing the date/time format of the unlocked achievements
* removed the condition which disabled the overlay sounds when it is shown
