Improve framerate warnings and show debugger for FlxG.warn()

- Changed warning messages for the flash framerate being higher than the game framerate. It wasn't gramatically correct (should've been "you are about to", not "you about to"), and I also think it's good practice to always include the source of the error (like `FlxG.framerate` or `FlxG.flashFramerate`
- set `openDebugger` to true for the warning `LogStyle` - this prevents warnings from being unnoticed. However, this also means there isn't much which distuingishes errors from warnings. I think we need a clearer ruleset for when to use errors and when to use warnings.
- Removed the left-over documentation for the `UseSystemCursor` param in the constructor of `FlxGame` for now. It might be worth the re-introduce it at some point though.