More consistent API for FlxTween / FlxPath / FlxTimer

`FlxTimer` and `FlxPath` (now consistent with tweens): 
- `abort()` -> `cancel()`
- `paused` -> `active`

Removed `userData` from tweens and timers. `function.bind()` should be used for that, and we are already discouraging the use of `Dynamic` and `cast` with the recent callback function changes.

`FlxTween.cancel()` now sets `finished` to true.