Use MutationObserver instead of DOMNodeInserted

Switched the game_container event handling to MutationObserver, replacing DOMNodeInserted which is deprecated.
**This will fix car selection in WebKit-based browsers.**
Event handling will fall back to DOMNodeInserted if MutationObserver isn't available.

There are a few more places where DOMNodeInserted is being used (game menu etc.), this still needs to be fixed.