FlxSlider refactor

Resubmit of #406 because of a merge conflict.

- removed `rightBorder` and `leftBorder` sprites
- update the `handle` position when the variable value is changed from outside (if no callback was set) resolving #402
- added `setCallback()`
- removed `Callback` param from constructor and added a bunch of others
- proper nulling of objects in `destroy()`
- make use of `FlxMath` for hovering logic
- changes to `createSlider()` make it easier to position sliders, their origin is now at 0,0 due to using `offset`
- added `clickable` and `hoverAlpha` vars
- use `FlxU.roundDecimal()` and drop the `round()` function 

Probably forgot a few more things. Essentially a refactor of the class.