Callback params for FlxButton, remove broken "on"-variable, closes #432

- added the option of having params for each of the callback functions, one of the main features `FlxButton` lacked compared to `FlxButtonPlus` in my opinion. Used via
 - `setOnUpCallback()`
 - `setOnDownCallback()`
 - `setOnOutCallback()`
 - `setOnOverCallback()`
- removed `on` for now, it seems rather broken - added a `TODO` though
- random improvements, like added documentation, usage of `?` for optional params and inlining a few things
- added missing `FlxAssets` import