FlxG.keyboard: Remove pressed, justPressed and justReleased to get rid of dynamic typing

- the "any"-functions provide the same functionality, just need to pass an array instead
- also makes the API clearer (the "or"-connection between params isn't immediately obvious for a function named "pressed()")
- less error-prone due to proper static typing instead of Dynamic arrays + makeVarArgs(), might also be faster
- removed ability to pass integers to checkKeyStatus(), there's checkStatus() for that already